import libtap, convert nodeset tests to TAP

Keeping it simple:
- imported libtap
- reworked nodeset_selftest() as a conditionally compiled test main with TAP calls instead of asserts
- test_nodeset is built with -DTEST_MAIN as a check_PROGRAMS
- Makefile.am TESTS executes it  (and others we might add) with 'make check', neatly summarizing results on modern automake, spewing the TAP output directly on the rhel6 one.