tests: Zero out transparent pixels before calculating digest.

The colour value of transparent pixels is irrelevant from a correctness
perspective, but we are nevertheless including that information in the
tests.  While we could zero out the values in the PAM output code it's
better to make the tests more robust.

This adds an external dependency on netpbm's pamarith and pamchannel
tools for the test suite.  Restructure the tests so that these tools
are optional.  If the tools are not available (or not working) then we
must accept the fact that images with an alpha channel may have false
negatives (but not false positives) -- record a SKIP result for an MD5
mismatch in that case.