Improve messages when test suites were not solvable

[ezyang]

I recently wrote a test suite, but typoed one of its dependencies, rendering it unbuildable. Despite there being a completely bogus test suite, the dependency solver happily passed the package, simply opting to disable test suites to make things work.

When I asked new-build to subsequently build the test, I got this message.

ezyang@sabre:~/Dev/backpack-str$ cabal new-build str-undefined:tests
cabal: Cannot build the test suites in the package str-undefined-0.1.0.0
because none of the components are available to build: the test suite
'implements' is not available because the solver did not find a plan that
included the test suites

This message is OK, but a better message would say how to find out what the solver error for test suites was. I think the correct way to do this is to rerun the command with --enable-tests.

[grayjay]

The discussion in #3923 is related.

[refaelsh]

@ezyang Exact same thing happened to me too.