testsuite: prefix handling

[Tom Tromey]

>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

Pedro> I'm also wondering if with_test_prefix should supply the `:'.
Pedro> gdb.trace/strace.exp
Pedro> seems to be the only test that doesn't append `:', but looking at it, it
Pedro> probably should.  We could add a with_raw_test_prefix or something like
Pedro> that that doesn't add neither the space nor the `:' if necessary.

It would be fine by me to add it.

Tom