"stack test --coverage" only generates reports for one testsuite

[mgsloan]

A known and intentional limitation of stack test --coverage is that it only produces a report for the last test-suite in the cabal file. This is a rather tricky problem involving the behavior of hpc:

The .mix files generated for the exeuctable's modules all go directly in the index directory .stack-work/dist/.../hpc/.hpc, rather than being in a subdir for that particular executable. This means that if there are modules named the same thing in each test-suite (e.g. Main), then Main.mix will get overwritten.

I think this can be resolved by specifying a different -hpcdir when building the actual tests, and then cleverly moving the files around when it's time to generate the reports. A previous attempt at this proved to be troublesome, though.

[mgsloan]

Due to the resolution of #634, it currently doesn't matter that these .mix files get overwritten, because they aren't part of the library. So, I've added a change lifting this limitation.

This issue will be back if #664 provides access to the coverage results of the actual test code.