Don't try to detect and use different compilers, since it's better to
specify this via the environment (e.g. via the Travis CI build matrix).
While doing this, also deduplicate the logic for testing with valgrind
and UBSAN, improve the log messages, and add a test with -O3.
Now that run_tests.sh has been cleaned up to remove (or move) test
groups that weren't very useful, remove the concept of test groups and
just run all the tests.
The reason that run_tests.sh supported running checksum_benchmarks.sh is
that as a side effect, checksum_benchmarks.sh runs the 'test_checksums'
program with all combinations of CPU features.
However, commit ec60cb48d11c ("tools/run_tests.sh: test different
combinations of CPU features") made run_tests.sh handle this elsewhere.
So having run_tests.sh run checksum_benchmarks.sh is no longer useful.
Keep checksum_benchmarks.sh around for manual benchmark runs, however.
android_tests is only useful for local testing, and it wasn't being run
in Travis CI. Move it into a separate script to avoid complicating
run_tests.sh.
This was only useful for me to do local testing, I don't have the needed
MIPS router available anymore, and its main purpose was test a big
endian system but that's now covered by testing s390x with Travis CI.
