ghc: fix running of optional testsuite

`brew install ghc --tests` emits a bunch of warnings and subsequently
fails because staging the "testsuite" resource makes its files
available in a `testsuite` subdirectory—*not* in the current
directory, which is where the formula *expects* them to be.

Closes #28330.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>