retest sounds nice; it would let you run the tests for shiny new Foo-1.23 against the installed Foo-1.2 before deciding to install the new version. (Yes, this has issues if 1.23 is a feature release, not just a bugfix release.)

With a retest action in place, I'm not sure there's a need for installing the tests; just keep the module build directory intact and run retest as desired. I like to do that anyway, with {config,make,test,install}.log files saved in it to preserve record of any problems during the original install.