Fix gdb.tui/completion.exp test

Fix gdb.tui/completion.exp test

When I run it locally, the test gdb.tui/completion.exp test fails
because of a timeout:

Running /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.tui/completion.exp ...
FAIL: gdb.tui/completion.exp: completion of layout names: tab completion (timeout)

The problem seems to be this regex, which confirms that after doing
layout<TAB>, "layout" is printed again after the gdb prompt:

  -re "^$input_line$"

The problem is that there's a trailing space in the output after
"layout".  Since the regex has an anchored end (the $), it doesn't
match.  Adding a space fixes the test.

gdb/testsuite/ChangeLog:

	* gdb.tui/completionn.exp (test_tab_completion): Add space in
	regex.