Added NPLM tests for applylm and hifst

*** This is an experimental feature in hifst/applylm, largely untested!
- These tests use inferno.nnlm file in NPLM package to assess
  that the ucam tools don't crash and score the lattices. ;)
- Regression tests can now be skipped (with echo 2). i.e.  NPLM not enabled, then these tests are skipped.
- Also, add NPLM lib dependency in tests, if available.