Basic unit testing for Fortran in a locked-down environment

What would be a sensible approach to take to try and add some basic unit tests to a large body of existing (Fortran 90) code, that is developee solely on a locked-down system, where there is no opportunity to install any 3rd party framework. I'm pretty much limited to standard Linux tools. At the moment the code base is tested at a full system level using a very limited set of tests, but this is extremely time consuming (multiple days to run), and so is rarely used during development

Ideally looking to be able to incrementally add targetted testing to key systems, rather than completely overhaul the whole code base in a single attempt.