PNG header in dummy file for use in testing.

$ python
Python 2.6.6 (r266:84292, Jan 22 2014, 09:42:36)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> magic = "".join(chr(x) for x in [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])
>>> with open("test-data/magic.png", "w") as h: h.write(magic)
...

Plan is to use this with the "contains" comparison offered by
the Galaxy test framework.
Relax mummerplot test to just check for PNG magic bytes
MUMmer's 'make install' is in place, need to add to PATH