gdb/testsuite: add return after a call to 'untested'

In gdb.base/corefile-buildid.exp, in the function
do_corefile_buildid_tests, if we fail to find the build-id for the
test binary then we call 'untested', but then push on with the test,
which inevitably fails as the rest of the test depends on having found
the build-id.

I think we're missing a 'return' after the call to 'untested' which
I've now added.

Also I noticed that we call build_id_debug_filename_get and then
manually remove '.debug' from the end.  This is no longer necessary,
we can just ask build_id_debug_filename_get to not add the suffix.