qtestlib selftests should test negative cases

The current qtestlib selftests mostly test positive cases, i.e. those in which testlib is expected to function successfully. The tests should be extended to also cover negative cases, such as tests being passed invalid or conflicting command line options.

Such tests should be added as one or more new test functions in tests/auto/testlib/selftests – the existing test function there is geared towards testing the logging behaviour, and the negative tests will generally not get as far as attempting to produce test logs, but rather will produce error messages on the console before aborting.