Should fix the issue reported in https://lists.osgeo.org/pipermail/proj/2020-January/009188.html
Some extra north-american grids present in data/ can affect the results of
some tests, so create a data/for_tests/ subdirectory in which we copy only
select grids.

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.