t/Makefile: run unit tests alongside shell tests

t/Makefile: run unit tests alongside shell tests

Add a wrapper script to allow `prove` to run both shell tests and unit
tests from a single invocation. This avoids issues around running prove
twice in CI, as discussed in [1].

Additionally, this moves the unit tests into the main dev workflow, so
that errors can be spotted more quickly. Accordingly, we remove the
separate unit tests step for Linux CI. (We leave the Windows CI
unit-test step as-is, because the sharding scheme there involves
selecting specific test files rather than running `make test`.)

[1] https://lore.kernel.org/git/pull.1613.git.1699894837844.gitgitgadget@gmail.com/

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>