Introduce an environment var to run "Test for dangling rules"

This test currently only runs on checkbuilds. To help with validations
of soong only packaging, introduce an environment variable to run this
test.

Test: RUN_BUILD_TESTS=true m nothing
Test: Verified that "Test for dangling rules" is in logs
Bug: 394096939
Bug: 393617060
Change-Id: I0fc5b38da6ff58fb952245b15073ed7015f9f5a6