option in buildtest build to not run tests but only generate the scripts

[shahzebsiddiqui]

Is your feature request related to a problem? Please describe.
It would be nice to view generated tests without running them. This would help troubleshoot builds before running them through the executor

Describe the solution you'd like
add option to buildtest build --stage=build which will build the testscripts without running them. Alternately, we can have
--stage=run which will run the test without having to build the test scripts. Right now we validate, build, and run the test. The --stage=run will run the test assuming it was already built. In order for this to work, buildtest will need to build a cache file for all builds and check if testscript exist for corresponding buildspec before running test.

@wspear