`copilot-bluespec`: Update copilot-bluespec.cabal to only run tests when passed "test-bluespec" flag. Refs #673.

[chathhorn-galois]

The copilot-bluespec package requires bsc (the Bluespec compiler) to be installed in order to run its test suite, but the builders that Hackage uses to run uploaded packages' test suites do not have bsc installed, causing Hackage to spuriously claim that copilot-bluespec's tests are failing.

This PR adds a cabal flag test-bluespec intended to enable tests that require the Bluespec compiler (bsc) to run. The default behavior of cabal test (with no flags) is now to not build the testsuite, preventing spurious test failures from appearing in Hackage reports. To enable building the testsuite, use: cabal test -f test-bluespec.

Fixes #673.