The tests weren't explicit enough about comparing under different
unique allocation strategies. This led to some confusion on my part when
I started overriding flags in `testsuite/mk/test.mk`.
Includes a `.gitignore` rule.

Test Plan: harbormaster

Reviewers: austin, simonmar, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1555