testsuite/T9379: Use GHC.Conc instead of Control.Concurrent.STM

Summary:
`GHC.Conc` provides almost everything we need.

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>

Test Plan:
    make test TEST=T9379

Reviewers: austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D152