Testing: Return failure from OK1() and other fixes.

* The tests for common and meta_data logged "not ok" but didn't signal
  failure because OK1() didn't include a return(-1) line. Adding this line
  caused some restructuring of the utils_vl_lookup test, because it used
  that macro in non-int functions.
* Fix DBLEQ() to work correctly with an expected NaN. Previously, the
  if condition would fall through to the "expect != actual" part, which
  is true for "NaN != NaN".
* Let the mock cdtime() return a non-zero value, as the (invalid) zero
  value is used in parse_values() to detect whether the time has been
  parsed already. This lead to the "N:..." tests failing.
* Correct the expected behavior of meta_data_add_*() when keys already
  exist: they're replaced rather than causing an error.