feat(ex_unit_test): support Windows - #5

ex_unit_test fails on Windows with fail("not implemented"). This writes a .bat runner there instead.

• It uses the Elixir and Erlang homes from the toolchain. A relative Elixir home is made absolute before the script changes directory.
• It needs an external Erlang. With an Erlang that has to be unpacked, it fails with "ex_unit_test on Windows needs an external Erlang".
• It keeps the check that fails a suite that ran no tests and excluded none.
• It adds a test-windows CI job for the ex_unit_test targets in test/, plus checks that the failing and empty tests fail.

Not run yet: the Windows job has not run, so the absolute path check and an external OTP toolchain on Windows are unproven. The Unix branch is unchanged.

Targets with elixir_app deps are left out of the Windows job. bin/elixir misreads paths under Git Bash, so elixirc would not work there.

Assisted by Claude.