Remove output files when first testing QMTP with MOPAC

If an output file exists from a previous run, the method
will to read the `.thermo` file printed from that
run, and not test the rdKit molecule generation nor the
MOPAC specific methods. Removing it ensures `testGenerateQMData`
covers these methods, while `testGenerateThermoData` will cover
the use of an existing `.thermo` file.
Check that your MOPAC installation is accessible

Checks that your MOPAC environment variables are correctly
declared.