[gdb/testsuite] Make Term::wait_for "" match only a prompt

The semantics of Term::wait_for is:
...
    # 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} {
...

Note that besides the regexp, also a subsequent gdb prompt is matched.

I recently used wait_for "" in a few test-cases, thinking that this would
match just a prompt, but in fact that's not the case.

Fix this in wait_for, and add a corresponding test in gdb.tui/tuiterm-2.exp.

Tested on x86_64-linux.