Previously the 'make check' test pngtest-all looked for given messages
at EOL. The match failed with Windows/MSYS2 because of the Windows
<cr><lf> line endings output by pngtest. This changes the test to look
for the message anywhere in a line; this might give false matches but
the specific messages being searched for are not likely to cause this
problem.
Signed-off-by: John Bowler <jbowler@acm.org>
8fb49b8b94