[gdb/testsuite] Fix gdb.server/unittest.exp for remote target

With test-case gdb.server/unittest.exp and a build with --disable-unit-tests I
get:
...
(gdb) builtin_spawn /data/vries/gdb/leap-15-4/build/gdbserver/gdbserver \
  --selftest^M
Selftests have been disabled for this build.^M
UNSUPPORTED: gdb.server/unittest.exp: unit tests
...
but with target board remote-stdio-gdbserver I get instead:
...
(gdb) builtin_spawn /usr/bin/ssh -t -l vries localhost \
  /data/vries/gdb/leap-15-4/build/gdbserver/gdbserver --selftest^M
Selftests have been disabled for this build.^M
Connection to localhost closed.^M^M
FAIL: gdb.server/unittest.exp: unit tests
...

Fix this by making the regexp less strict.

Tested on x86_64-linux.