testsuite fixes and minor enhancements

A couple things were missing from the testsuite code in my earlier pull request, most notably the
[README]. I also add a couple more sanity tests and a check for `test_under_flux` functionality.

[README]: https://github.com/grondo/flux-core/blob/5f912048d9d5605f797fc3513cd265b6786090d3/t/README.md