tests/pngstest-* are changed so that the new test files are divided into 8
groups by gamma and alpha channel.  This makes each test take just under 5
seconds on a moderately up-to-date x86_64 system using libpng16 (test times are
somewhat variable under libpng17 at present because of the lack of optimization
in some gamma processing paths.)

These tests have considerably better code and pixel-value coverage than
contrib/pngsuite however coverage is still incomplete.
Signed-off-by: NJohn Bowler <jbowler@acm.org>