[gdb/testsuite] Detect gdb prompt after monitor exit

With this gdbserver-support.exp patch:
...
@@ -451,8 +451,10 @@ proc gdbserver_exit { is_mi } {
 	    # We use expect rather than gdb_expect because
 	    # we want to suppress printing exception messages, otherwise,
 	    # remote_expect, invoked by gdb_expect, prints the exceptions.
+	    set have_prompt 0
 	    expect {
 		-i "$gdb_spawn_id" -re "$gdb_prompt $" {
+		    set have_prompt 1
 		    exp_continue
 		}
 		-i "$server_spawn_id" eof {
@@ -463,6 +465,7 @@ proc gdbserver_exit { is_mi } {
                    warning "Timed out waiting for EOF in server after $monitor_exit"
                }
 	    }
+	    gdb_assert {$have_prompt}
 	}
     }
     close_gdbserver
...
and with this in parallel:
...
$ stress -c 5
...
we run into this and similar FAILs:
...
FAIL: gdb.multi/multi-target.exp: continue: non-stop=on: $have_prompt
...

In more detail:
...
(gdb) PASS: gdb.multi/multi-target.exp: continue: non-stop=on: inferior 5
Remote debugging from host ::1, port 40712^M
Process build/gdb/testsuite/outputs/gdb.multi/multi-target/multi-target \
  created; pid = 11098^M
monitor exit^M
Killing process(es): 11098^M
FAIL: gdb.multi/multi-target.exp: continue: non-stop=on: $have_prompt
spawn build/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory \
  build/gdb/testsuite/../data-directory^M
...

After issuing a "monitor exit" command, we should always get a prompt back, so
check for that.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-09-16  Tom de Vries  <tdevries@suse.de>

	* lib/gdbserver-support.exp (gdbserver_exit): Make sure we
	get the gdb prompt after issuing "monitor exit".

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-09-16  Tom de Vries  <tdevries@suse.de>
+
+	* lib/gdbserver-support.exp (gdbserver_exit): Make sure we
+	get the gdb prompt after issuing "monitor exit".
+
 2020-09-16  Tom de Vries  <tdevries@suse.de>
 
 	PR testsuite/26624

gdb/testsuite/lib/gdbserver-support.exp

@@ -451,13 +451,20 @@ proc gdbserver_exit { is_mi } {
 	    # We use expect rather than gdb_expect because
 	    # we want to suppress printing exception messages, otherwise,
 	    # remote_expect, invoked by gdb_expect, prints the exceptions.
+	    set have_prompt 0
 	    expect {
 		-i "$gdb_spawn_id" -re "$gdb_prompt $" {
-		    exp_continue
+		    set have_prompt 1
+		    if { [info exists server_spawn_id] } {
+			exp_continue
+		    }
 		}
 		-i "$server_spawn_id" eof {
 		    wait -i $expect_out(spawn_id)
 		    unset server_spawn_id
+		    if { ! $have_prompt } {
+			exp_continue
+		    }
 		}
                timeout {
                    warning "Timed out waiting for EOF in server after $monitor_exit"