maketx fix -- don't clobber the ImageDescription with the hash

Have maketx more robustly remove "SHA-1=..." and "ConstantColor=..." from the ImageDescription of the input image, without losing whatever else may be in that metadata.

Also added testsuite/maketx so that we can have little tests for all the edge cases of maketx.  To start with, I added a test for this particular maketx change.