Use absolute error instead of average error for path check
introspect: Print full precision for the path error
Use a static set of reference pixels

The ref pixels are stored in babl/babl-ref-pixels.inc which
is generated by tools/babl-gen-test-pixels.
Remove old references to introspection
Add babl-ref-pixels.inc to EXTRA_DIST
Add automake 1.13+ test outputs to .gitignore
Disable babl-gen-test-pixels when random is missing

babl-gen-test-pixels is only needed to regenerate the source
pixels and doesn't need to be run as part of the normal build
process.
tests: Use rand when random is unavailable