Mac OS X testing enhancements

[cekees]

• [ ] replace whole file/subdir skipping with finer-grained pytest skip statements
• [ ] remove logic in makefile once os command line looks same as linux
• [ ] work through skipped tests to re-enable on os x
• [ ] think about whether there are any os x or linux-specific tests that we should create and tag

My view is that this is low-priority for now.