Add short-circuit logic and convenience tempfile functions

[johnwikman]

This PR adds the following convenience functions:

• Short-circuit logic in bool.mc. This is useful to perform a sequential evaluation only if the previous one failed. The use case is to check that a command exists prior to running that command in the test spec, but that is now moved to its own PR.
• Adds the sysWithTempDir and sysWithTempFile in sys.mc that emulates the python with tempfile.TemporaryFile scope that automatically cleans up the file or directory once we are done with it.