Two test updates that didn't make it in #104

[ceblanton]

• Skip the Test04 parallel tests only if requested (i.e. potentially run through CI)
• Compare the serially and parallel fregrid output

[ceblanton]

@ngs333, this adds a nccmp check for the parallel part of Test15 and uses different (improved?) logic for skipping the slow Test04 on the github runners (as it's way too slow). I wanted to allow the option of running the parallel part of Test04 through CI and skip it in the github runners. It works, but not sure it's a good approach. @rem1776 what do you think?

[rem1776]

@ceblanton Sorry I guess I missed that nccmp, but looks great, it's probably much better to skip it in the workflow file rather than check that variable in the script since it can run elsewhere. I guess it'll be skipping the whole test for mpi jobs not just the parallel section but that should be fine since the non-mpi jobs are running it anyway.