Summary:
The aggressive cleanup routine of T1407 is removing files that don't belong to it.
Constrain the test to only removing files it should by putting all it's generated
binaries in it's own output folder.

Test Plan: make test -C testsuite/tests/ghci/linking/dyn

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2165

GHC Trac Issues: #1407