check-all: Allow convenience --gtest_filter passthrough

Technically the harness allows additional arguments after a lone '--' to
be passed to the test binary itself. Since the --gtest_filter option is
so common, it's easier to just avoid the '--' if possible, altogether.

Change-Id: I88a320df3b82317a66cf2f7cd252c206d4119277
Reviewed-on: http://review.couchbase.org/41446
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>