Make lua-releng return non-zero return code if it finds a problem

This commit makes the following improvements to lua-releng:

1. Adds a -silent command-line parameter (it must be the first parameter
   on the command-line) which suppresses output (other than failures)

2. Makes it only look for a t/ directory if not files have been specified
   on the command-line

3. Removes commented-out and unused code

4. Calls 'die' whenever a problem is encountered so that this can be used
   in a test suite to fail if there are problems.