make: all-am should build tests, pngvalid: test filters
Somewhere down the line 'make all-am' stopped building the tests, it's probably
a mistake anyway to make all-am, but it worked before.  Now the tests are built
again.
pngvalid now generates size images with as many filters as it can manage,
limited by the number of rows.

Signed-off-by: John Bowler <jbowler@acm.org>