Support for application unit tests
Based on a recent discussion, fab should support application unit tests e.g. tests using the pfunit framework.
This need to be a bit more investigation to decide if this is a Fab or a BAF issue. There is already https://github.com/MetOffice/lfric-baf/issues/18 (assuming that all we need to do for unit tests is to do an additional phase before the rest of fab can start), but if we need a 'deeper' support for pfunit, it might need to be done here.
This needs support for multiple modules per file and external keyword in fortran - see #305 and #271 for details - but no additional fab support is needed beyond this, everything else is a build system issue.