gitlab-ci.yml: Only run one test-case per fuzzer

gitlab-ci.yml: Only run one test-case per fuzzer

With 1000 runs, there is a non-negligible chance that the fuzzer can
trigger a crash. With this CI job, we care about catching build/runtime
issues in the core fuzzing code. Actual device fuzzing takes place on
oss-fuzz. For these purposes, only running one input should be
sufficient.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201002143524.56930-1-alxndr@bu.edu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>