Add usage to run-libcore-tests.sh

Update command-line parsing to add sanity checks to choice of mode and
emit usage if an error is found or --help is specified as an argument.

Test: art/tools/run-libcore-tests.sh --mode host
Change-Id: I93d853fdfb299b65dc0e55be4a22d9a7f7a95d79
1 file changed