utils.py testing framework improvements

[AndrewTolmach]

A set of changes to tidy up the testing framework and add new functionality:

In utils.py:

• flag for emulation of final x86 program is exposed in a new function
• exceptions are caught and printed before continuing, allowing easier testing of large suites.
• no .in file is needed for tests that don't do input
• executable files are saved with .exe extension rather than defaulting to a.out
• new entry point for executing a selected list of files

There is a new file run-some-tests.py that allows a set of tests to be specified on the command line using standard shell expansion, and provides flags for setting tracing and/or emulation.