gdb.base/corefile-buildid.exp: fix DUPLICATEs when failing to generate a core file

When my system isn't properly configured to generate core files in the
local directory, I see these DUPLICATEs:

    DUPLICATE: gdb.base/corefile-buildid.exp: could not generate core file

Fix that by having a single with_test_prefix around that message and
what follows.

Change-Id: I4ac245fcce1c666db56e3bad3582aa17f183dcba