bin/modernish:
- Store the path to the test sets (libexec/modernish/tests) in a
  'testsdir' variable so, if it ever changes, it only needs to be
  updated in one location.

libexec/modernish/tests/run.sh:
- Once again show the full path to the test set script in the
  default output headers, for copy-and-paste purposes in case
  someone wants to view or edit the tests.
- The canonical name of the test set (for use with the -t option)
  is now highlighted in red for clarity.
- Change hardcoded paths to test sets to $testsdir.