compareFstats.c: more robust comparison for small F-values

- use '4' as smallest reference value for relative differences
Revert "LALPulsar: remove LAL_NDEBUG/lalNoDebug conditionals from tests"

This reverts commit d69ba4116e7d26d970cd263b66113591c856cb27.
The test otherwise fails if NDEBUG is defined.
some cleanup in test scripts testCFSv2[_resamp].sh

- use consistent subdir for each test's temporary files