TEST_MAPPING: add new test target to postsubmit

Part two of three in order to rename a test target. Needed for SLO numbers
before we can move to presubmit for step 3.

Test: cd external/llvm-libc && atest :postsubmit
Link: go/test-mapping-slo-guide
Change-Id: Idb15a47c86fb8b337506297b32985d3d17f97846