Fix sa tests.

Small divergences in the output padding made some sa tests fail.  Just
trim all whitespace from the outputs and the golden files so comparisons
are less fragile and the tests pass again.
Turn a test precondition into a skip in the mdconfig tests.

Tests that cannot be run because a precondition is not met should be
marked as skipped, not failed.  Do this for the tests in mdconfig that
first check if the caller user is root.
Make the strerror tests work without libtap.

Just replace the simple calls to the library with ad-hoc code.  We should
later rewrite these with the ATF libraries anyway, which are part of the
base system.
Remove broken tests for eui64_line.

This function is not public and brooks (initial committer adding the code)
suggests the deletion of the tests (which I don't know if they work)
instead of changing the visibility of the function.