* Refactor unit test scripts.
Does the following:
1. Remove as many dependencies on the operating system shell as possible.
For example, use of shutil.rmtree(...) instead of os.system('rm -r ...').
This brings this script a bit closer to being able to run on Windows.
2. Switch from os.system() to subprocess.check_call().
* This is a bit more secure as check_call() directly invokes the subprocess
without evaluation the arguments on a command-line.
* Removes the need to evaluate the return code as check_call() does this.
* Can directly set environment variables (e.g. env=cmd_env) instead of
including with subprocess invocation (e.g. BIN=test.bin).
3. Minor cleanup to main.py sys.argv parsing.
4. PEP8 formatting.
* Ignore FileNotFoundError for rmtree('report').
* Back to os.system for gcovr.
* Removed unused shutil import.
