[ci] Make test.yml a reusable workflow

Change test.yml to be a reusable workflow so that this can be called with
different inputs (different matrix arguments) for different test
scenarios.  One PR scenario is testing PRs, branches, and releases.  This
is added in the ci.yml workflow.  The other, future scenario is nightly
testing with CIRCT.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>