test-tool run-command testsuite: support unit tests

test-tool run-command testsuite: support unit tests

Teach the testsuite runner in `test-tool run-command testsuite` how to
run unit tests: if TEST_SHELL_PATH is not set, run the programs directly
from CWD, rather than defaulting to "sh" as an interpreter.

With this change, you can now use test-tool to run the unit tests:
$ make
$ cd t/unit-tests/bin
$ ../../helper/test-tool run-command testsuite

This should be helpful on Windows to allow running tests without
requiring Perl (for `prove`), as discussed in [1] and [2].

This again breaks backwards compatibility, as it is now required to set
TEST_SHELL_PATH properly for executing shell scripts, but again, as
noted in [2], there are no longer any such invocations in our codebase.

[1] https://lore.kernel.org/git/nycvar.QRO.7.76.6.2109091323150.59@tvgsbejvaqbjf.bet/
[2] https://lore.kernel.org/git/850ea42c-f103-68d5-896b-9120e2628686@gmx.de/

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