For reasons unknown to anyone but the geniuses from Redmond: In
MSWindows, the wildcard pattern "*.gie" expands to all files having
an extension *starting* with ".gie".

In other words "dir *.gie" will list files with extension ".gie"
*and* with extension ".gie_failing".

This means that running "gie test\gigs\*.gie" will run all gigs
tests, even the ones known to fail.

So rather than getting a quick check for regressions, one gets
a lot of noise and no easy way to be sure whether any regressions
have appeared.

To work around this oddity, we rename all files ending in
".gie_failing" to end in ".gie.failing" - and while at it,
adding all the non-failing gigs tests to the AppVeyor test suite.

This extends the regression test suite, run at the end of each
AppVeyor integration, to over 2500 individual tests.

Test coverage, however, is still hovering just below 75%, so
there's still plenty of room and reason for additional tests.

All the GIGS JSON files have been converted to the gie format
and are now covered by running "make check". Therefore we no
longer need the Python setup. Removing it from the Travis CI
setup should speed up the test execution time a bit.

Make sure that the gie files in test/gie and test/gigs are included
in the distribution tarball. Tests with gie are set up to run all
available files matching *.gie in the gie and gigs directories. For
this reason tests that are known to fail in gigs/ have been renamed
to *.gie_failing, so that "make check" can be run in-tree in local
copies of the git repository.