[gdb/testsuite] Update infrun regexp in gdb.base/watch_thread_num.exp

The test-case gdb.base/watch_thread_num.exp contains an infrun regexp:
...
       -re "infrun:" {
...
which doesn't trigger because:
- the test-case doesn't contain "set debug infrun 1", and
- if we hack the test-case to add this, the regexp doesn't match
  because "[infrun] " is printed instead.

Make the test pass with "set debug infrun 1" and add the setting commented
out.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-05-11  Tom de Vries  <tdevries@suse.de>

	* gdb.base/watch_thread_num.exp: Fix "set debug infrun 1" FAILs.
	Add "set debug infrun 1" commented out.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-05-11  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.base/watch_thread_num.exp: Fix "set debug infrun 1" FAILs.
+	Add "set debug infrun 1" commented out.
+
 2021-05-11  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.base/gdb-sigterm.exp: Fix exp_continue regexp.

gdb/testsuite/gdb.base/watch_thread_num.exp

@@ -81,18 +81,30 @@ gdb_test "info breakpoint \$bpnum" \
     "stop only in thread $thread_num" \
     "info breakpoint shows watchpoint is thread-specific"
 
+# Uncomment to see additional information.
+#gdb_test "set debug infrun 1"
+
 for {set i 1} {$i <= 5} {incr i} {
     set watchpoint "Watchpoint triggered iteration $i"
     set check "Check thread that triggered iteration $i"
 
     set test $watchpoint
+    set seen_watchpoint 0
     gdb_test_multiple "continue" $test {
-	-re "infrun:" {
-	    # Avoid timeouts when debugging GDB.
+	-re "Hardware access \\(read/write\\) watchpoint .*: shared_var" {
+	    set seen_watchpoint 1
 	    exp_continue
 	}
-	-re "Hardware access \\(read/write\\) watchpoint .*: shared_var.*$gdb_prompt $" {
-	    pass $test
+	-re "$gdb_prompt " {
+	    if { $seen_watchpoint } {
+		pass $test
+	    } else {
+		fail $test
+	    }
+	}
+	-re "\\\[infrun\\\] " {
+	    # Avoid timeouts when debugging GDB.
+	    exp_continue
 	}
     }
     gdb_test "thread" ".*Current thread is $thread_num .*" $check