make test_shim work when in non-home, or symlinked directories.

* on supercomputer, home is a symlinked path.
* shim_tests use realpath, and assume home is a real path, so some tests fail.

purpose is to get the tests to pass when run on supercomputer.

branches with bits of work on this:

* https://github.com/MetPX/sarrac/tree/issue133_non_utf8_headers

which was merged from:

* https://github.com/MetPX/sarrac/tree/shim_test_automation

the non_utf8 and test_shim failures in the normal cases have since been isolated and merged, and so the work here is to extract the symlink related patches and get them into something coherent to merge, so in future, people can run the test_shim on the supercomputer.