Plugin tests: Allow to specify which test functions to run

This simplifies debugging single test functions.

Test data can be specified the same way as for QTest executables.

New syntax for the -test option:
    -test <plugin> [testfunction[:testdata]]...

Examples:
    ./qtcreator -test Git testDiffFileResolving
    ./qtcreator -test Git testDiffFileResolving testStatusParsing:"DU"

Change-Id: Ifea6b114bfc0fabe3e9ddffcc2fd90af157052ec
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>