Add sdkextensions tests to soong TEST_MAPPING

Changes in soong have left sdkextensions tests broken for over 2 days,
so let's prevent that happening again.

Bug: 180105615
Test: presubmit
Change-Id: I6961228bdd5a4606eb5078ed332b508ef6d2d6af
Merged-In: I6961228bdd5a4606eb5078ed332b508ef6d2d6af