[gdb/testsuite] Fix gdb.tui/scroll.exp with read1

When running test-case gdb.tui/scroll.exp, I get:
...
Box Dump (80 x 8) @ (0, 0):
    0 $17 = 16
    1 (gdb) p 17
    2 $18 = 17
    3 (gdb) p 18
    4 $19 = 18
    5 (gdb) p 19
    6 $20 = 19
    7 (gdb)
PASS: gdb.tui/scroll.exp: check cmd window in flip layout
...
but with check-read1 I get instead:
...
Box Dump (80 x 8) @ (0, 0):
    0 (gdb) 15
    1 (gdb) p 16
    2 $17 = 16
    3 (gdb) p 17
    4 $18 = 17
    5 (gdb) p 18
    6 $19 = 18
    7 (gdb) p 19
FAIL: gdb.tui/scroll.exp: check cmd window in flip layout
...

The "p 19" command is handled by Term::command, which sends the command and then
does Term::wait_for "^$gdb_prompt [string_to_regexp $cmd]", which:
- matches the line with "(gdb) p 19", and
- tries to match the following prompt "(gdb) "

The problem is that scrolling results in reissuing output before the "(gdb) p
19", and the second matching triggers on that.  Consequently, wait_for no
longer translates gdb output into screen actions, and the screen does not
reflect the result of "p 19".

Fix this by using a new proc wait_for_region_contents, which in contrast to
wait_for can handle a multi-line regexp.

Tested on x86_64-linux with make targets check and check-read1.

gdb/testsuite/gdb.tui/scroll.exp

@@ -60,7 +60,13 @@ Term::command "winheight cmd 8"
 Term::check_box "src window after resize" 0 8 80 16
 
 for {set i 10} {$i < 20} {incr i 1} {
-    Term::command "p $i"
+    set cmd "p $i"
+    send_gdb "$cmd\n"
+    Term::wait_for_region_contents 0 0 80 8 \
+	[multi_line \
+	     "$gdb_prompt [string_to_regexp $cmd]\\s+" \
+	     "\\\$\\d+ = $i\\s+" \
+	     "$gdb_prompt "]
 }
 
 # Now check that the contents of the command window are as expected.

gdb/testsuite/lib/tuiterm.exp

@@ -663,18 +663,10 @@ namespace eval Term {
 	_clear_lines 0 $_rows
     }
 
-    # Accept some output from gdb and update the screen.  WAIT_FOR is
+    # Accept some output from gdb and update the screen.
-    # a regexp matching the line to wait for.  Return 0 on timeout, 1
+    # Return 1 if successful, or 0 if a timeout occurred.
-    # on success.
+    proc accept_gdb_output { } {
-    proc wait_for {wait_for} {
 	global expect_out
-	global gdb_prompt
-	variable _cur_col
-	variable _cur_row
-
-	set prompt_wait_for "$gdb_prompt \$"
-
-	while 1 {
 	gdb_expect {
 	    -re "^\[\x07\x08\x0a\x0d\]" {
 		scan $expect_out(0,string) %c val
@@ -705,6 +697,24 @@ namespace eval Term {
 	    }
 	}
 
+	return 1
+    }
+
+    # Accept some output from gdb and update the screen.  WAIT_FOR is
+    # a regexp matching the line to wait for.  Return 0 on timeout, 1
+    # on success.
+    proc wait_for {wait_for} {
+	global gdb_prompt
+	variable _cur_col
+	variable _cur_row
+
+	set prompt_wait_for "$gdb_prompt \$"
+
+	while 1 {
+	    if { [accept_gdb_output] == 0 } {
+		return 0
+	    }
+
 	    # If the cursor appears just after the prompt, return.  It
 	    # isn't reliable to check this only after an insertion,
 	    # because curses may make "unusual" redrawing decisions.
@@ -724,6 +734,23 @@ namespace eval Term {
 	return 1
     }
 
+    # Accept some output from gdb and update the screen.  Wait for the screen
+    # region X/Y/WIDTH/HEIGTH to matches REGEXP.  Return 0 on timeout, 1 on
+    # success.
+    proc wait_for_region_contents {x y width height regexp} {
+	while 1 {
+	    if { [accept_gdb_output] == 0 } {
+		return 0
+	    }
+
+	    if { [check_region_contents_p $x $y $width $height $regexp] } {
+		break
+	    }
+	}
+
+	return 1
+    }
+
     # Like ::clean_restart, but ensures that gdb starts in an
     # environment where the TUI can work.  ROWS and COLS are the size
     # of the terminal.  EXECUTABLE, if given, is passed to
@@ -940,15 +967,23 @@ namespace eval Term {
     # and HEIGHT match REGEXP.  This is like check_contents except
     # only part of the screen is checked.  This can be used to check
     # the contents within a box (though check_box_contents is a better
-    # choice for boxes with a border).
+    # choice for boxes with a border).  Return 1 if check succeeded.
-    proc check_region_contents { test_name x y width height regexp } {
+    proc check_region_contents_p { x y width height regexp } {
 	variable _chars
 	dump_box $x $y $width $height
 
 	# Now grab the contents of the box, join each line together
 	# with a '\r\n' sequence and match against REGEXP.
 	set result [get_region $x $y $width $height "\r\n"]
-	gdb_assert {[regexp -- $regexp $result]} $test_name
+	return [regexp -- $regexp $result]
+    }
+
+    # Check that the region of the screen described by X, Y, WIDTH,
+    # and HEIGHT match REGEXP.  As check_region_contents_p, but produce
+    # a pass/fail message.
+    proc check_region_contents { test_name x y width height regexp } {
+	set ok [check_region_contents_p $x $y $width $height $regexp]
+	gdb_assert {$ok} $test_name
     }
 
     # Check the contents of a box on the screen.  This is a little