Patch from JF: Readability : Updated comments and improved naming for items
 in DiagramCreator.  Changed TestData so that the default
 action is to run-all-tests.
Experimental change which switches behaviour of TestCode.java.
Revert "Experimental change which switches behaviour of TestCode.java."

This reverts commit e32ea51b3c9fa4c7968a5fd63bfdf11069cbd0d7.
Removed occurrences of makeForTesting, which are no longer needed now
that we use JSON to convert from Strings to AbstractDescription.  The
testing framework persists in using old-style String syntax, which is
converted to JSON-style syntax before converting to
AbstractDescriptions.
Ensure that we can't create a null diagram