The runtests.sh script is removed and replaced with crosstest_generator.py.

"make check" limits to a relevant subset of cross tests to control the combinatorial explosion.  We cut the native tests almost in half, and the sandboxed tests down to a quarter.

The --include and --exclude logic is copied/adapted from szbuild.py.

The script works by running through every possible test in the combinatorial explosion, and if the test is a match against the --include and --exclude arguments, the test is built and run.

The script includes lit support, which is the most likely way it will be run.  When run with the --lit argument, it sprays the output directory with lit test files in the form of shell scripts, and "make check" runs lit on that directory.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4085
R=jvoung@chromium.org, mtrofin@chromium.org

Review URL: https://codereview.chromium.org/987503004