Set USE flags for igt-gpu-tools test binaries

Set the `tests` USE flag for igt-gpu-tools to ensure that the test
executables will be built and present in dev and test images.

BUG=none
TEST=emerge-$BOARD igt-gpu-tools and confirm test executables
     were built

Change-Id: I4f1a65e672979d97bf69551b7130d2b607a2966b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2278032
Tested-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Drew Davenport <ddavenport@chromium.org>
1 file changed