doc: test.py should report a different status when generating the command line documentation

When generating the introspected Doxygen docs we have test.py run each example with the NS_COMMANDLINE_INTROSPECTION environment variable set. This causes CommandLine to write out the usage (help) message to a file, which then gets included in the Doxygen for the example program. Once this is written the example exits with 0 status (success).

This results in a line from test.py summarizing the "test":

[0/463] PASS: TestSuite core-example-simulator

It would be nicer/more informative if the "PASS: TestSuite" was instead "Wrote example usage for"