Warn users that Inkscape must be in the path when testing Python extensions

[jazzynico]

Follow-up to Bug #1419266 "error running gimp_xcf.test.py in trunk".

Attached is a list of other Python extensions that probably would fail, or should fail, in a similar way if we were to try to create test routines for them. These are routines that attempt to open a new instance of Inkscape using Popen:

* dimension.py,
* generate-voronoi.py,
* guillotine.py,
*jessyInk_export.py,
* perspective.py,
* restack.py,
* summersnight.py,
* synfig_prepare.py,
* text_extract.py,
* text_merge.py,
* webslicer_export.py.

The test routines for perspective.py, restack.py, summersnight.py actually do exist and do not fail, but that is only because the tests are not sufficiently realistic, do not actually execute any significant code.

The file jessyInk_export.py is interesting because it contains a routine called 'findInkscapeCommand' which attempts to find inkscape by searching a few standard locations. However it would fail on my development machine.

I guess what I'm saying is it might be worthwhile to have a fairly generic warning up front in the test procedure to indicate that it is necessary to have Inkscape in the path, since this is not unique to gimp_xcf.py"