[gdb/testsuite] Make prompt matching in Term::wait_for more strict

On x86_64-freebsd, I run into:
...
Box Dump (80 x 24) @ (0, 0):
    0 (gdb) maint info screen
    1 Number of characters gdb thinks are in a line is 90.
    2 Number of characters readline reports are in a line is 89.
    3 Number of characters curses thinks are in a line is 90.
    4 Number of characters environment thinks are in a line is 90 (COLUMNS).
    5 Number of lines gdb thinks are in a page is 40.
    6 Number of lines readline reports are in a page is 40.
    7 Number of lines curses thinks are in a page is 40.
    8 Number of lines environment thinks are in a page is 40 (LINES).
    9 Readline wrapping mode: readline (terminal is not auto wrap capable, last column
   10 .
   11 (gdb) tui disable
   12 (gdb) tui disable
   13 (gdb) maint info screen
   14
   15
   16
   17
   18
   19
   20
   21
   22
   23
FAIL: gdb.tui/resize-2.exp: again: curses width 80
...

The problem is that the prompt matching in Term::wait for is not strict enough.

It will accept a line:
...
(gdb) foo
...
as long as the cursor is pointing just after the prompt, like so:
...
(gdb) foo
      ^
...

Fix this by also checking that the line is empty.

Tested on x86_64-linux.

gdb/testsuite/gdb.tui/tuiterm-2.exp

@@ -106,6 +106,48 @@ with_override Term::accept_gdb_output test_accept_gdb_output {
 		}
 		gdb_assert { [Term::wait_for ""] }
 	    }
+
+	    with_test_prefix "Term::wait_for 2" {
+		Term::_setup 4 20
+		set send_cnt 0
+		set expect_send {}
+		set action_cnt 0
+		set actions {
+		    {
+			Term::_move_cursor 0 0
+
+			Term::_insert "${::border}(gdb) "
+			set pos $Term::_cur_col
+
+			Term::_insert "foo"
+
+			Term::_move_cursor 19 0
+			Term::_insert "$::border"
+
+			Term::_move_cursor $pos 0
+		    }
+		    {
+			Term::_move_cursor 0 1
+
+			Term::_insert "${::border}(gdb) "
+			set pos $Term::_cur_col
+
+			Term::_move_cursor 19 1
+			Term::_insert "$::border"
+
+			Term::_move_cursor $pos 1
+		    }
+		}
+
+		# Wait for a prompt.
+		gdb_assert { [Term::wait_for ""] }
+
+		# The first action sets the cursor after the prompt on the
+		# first line.  The second action sets the cursor after the
+		# prompt on the second line.   Check that wait_for returns
+		# after the second action, not the first.
+		gdb_assert { $Term::_cur_row == 1 }
+	    }
 	}
     }
 }

gdb/testsuite/lib/tuiterm.exp

@@ -864,16 +864,32 @@ namespace eval Term {
 	    } else {
 		set prev [get_line $_cur_row]
 	    }
-	    if {[regexp -- $wait_for $prev]} {
-		debug_tui_matching "$fn: match: '$prev'"
-		if {$wait_for == "$prompt_wait_for"} {
-		    break
-		}
-		set wait_for $prompt_wait_for
-		debug_tui_matching "$fn: regexp prompt: '$wait_for'"
-	    } else {
+
+	    if { ![regexp -- $wait_for $prev] } {
 		debug_tui_matching "$fn: mismatch: '$prev'"
+		continue
 	    }
+
+	    if {$wait_for == "$prompt_wait_for"} {
+		# We've detected that the cursor is just after the prompt.
+		# Now check that there's nothing else on the line.
+		set prev [get_line $_cur_row]
+		if { ![regexp -- "(^|\\|)$gdb_prompt +($|\\||\\+)" $prev] } {
+		    debug_tui_matching "$fn: mismatch: '$prev'"
+		    continue
+		}
+	    }
+
+	    debug_tui_matching "$fn: match: '$prev'"
+
+	    if {$wait_for == "$prompt_wait_for"} {
+		# Matched the prompt, we're done.
+		break
+	    }
+
+	    # Now try to match the prompt.
+	    set wait_for $prompt_wait_for
+	    debug_tui_matching "$fn: regexp prompt: '$wait_for'"
 	}
 
 	return 1