Add --enable-tests for cabal test as well

[sorki]

Force tests despite plan not including them.

There something weird going on as w/o this I'm getting

cabal: Cannot test the package implicit-0.3.0.1 because none of the components
are available to build: the test suite 'test-implicit' is not available
because the solver did not find a plan that included the test suites. Force
the solver to enable this for all packages by adding the line 'tests: True' to
the 'cabal.project.local' file.

despite it working when I try it locally with the same Cabal version.

I think haskell/cabal#3923 is related but it might be something else, didn't dig too deep as it looks fine to just force that.

For the record here's the branch with my attempts:
https://github.com/sorki/ImplicitCAD/actions?query=branch%3Aactions