Add a -datatags command-line switch to QTestLib

Running a QTestLib program with a -datatags switch changes the behavior as follows:

• The test functions themselves are not invoked. Thus, any output normally generated from inside a test function will be omitted.
• The data tags for which each test function is invoked are included in the output.

One use case of this is to quickly compare the list of available data tags against the list of data tags for which results have actually been reported in the BM database.