Share *_OUT_TESTCASES environment variables with TF

Instead of having test tools search the OUT_DIR for the
testcases folders (of which there can be multiple different
targets), let's have these variables become part of the runtime
environment after a developer has run lunch.

Bug: 35322559
Test: . build/envsetup.sh ; lunch ; printenv | grep TESTCASES
Change-Id: Ic4addc2b9727ff717b2e595b9be224650e2f1b1d