ci: handle failures of test-slice helper

ci: handle failures of test-slice helper

The "run-test-slice.sh" script executes the test helper to slice up
tests passed to it. As the execution is part of a pipe though, we end up
ignoring any potential error code returned by the helper.

Make the code more robust by storing the tests in a variable first so
that we can split up the pipeline.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>