Rewrite the package test suite.
I've rewritten the test suite to be more concise and "correct by construction". The primary method that this is achieved by is introducing the 'TestM' monad, which carries around the important state for the tests so that (1) we don't have to pass it as an argument all around, and (2) we can automatically make the correct decisions about how to do things. This new method emphasises "configuration by convention": we assume that a test-case named "Foo" has its test packages in the directory "tests/PackageTests/Foo". A secondary change is that all command functions automatically fail if they have a non-zero exit code (unless you use the 'shouldFail' combinator which inverts the sense.) This saves a lot of typing on test-cases. (In fact, I've reorganized all of the commands related here.) In the process, I've tightened up the logic for how to find the LocalBuildInfo of the Cabal we've testing, so we should now reliably be testing the inplace Cabal library, and not the system library (as was often the case.) Because things are a lot shorter, there is no good reason to make Check modules except for the biggest test cases. Most test-cases have been folded into PackageTests.Tests; if you have a small test-case you should just put it there. Signed-off-by: Edward Z. Yang <email@example.com>