Cleanup test framework string formatting

* Use format strings instead of string concatenation.
* Wrap `config.compiler`, `config.hpc` etc. in quotes in `mk/test.mk`, so we
  don't have to in .T scripts and driver/testlib.py.

Update hpc submodule (test cleanup)

Reviewers: austin

Differential Revision: https://phabricator.haskell.org/D718