Run at least a few release-build tests on Travis

In light of a serious regression crashing all drcachesim/drmemtrace uses in #3478 (closed) which was not detected pre-commit but in our post-commit CDash tests, this issue proposes adding at least some subset of tests to be run in release build on Travis. The reason we don't run all of them is to keep the pre-commit time down to something reasonable.

I'm proposing giving a few key tests "TRAVIS" labels and run with "-L TRAVIS". We're already doing the release build on Travis so a few tests won't add too much time.

For reference on how long it takes to build vs run tests, here are the numbers on my CDash Linux machine:

debug-internal-64 build: 1:54 tests: 10:28

release-external-64 build: 1:42 tests: 6:13