Reduce length of TAP test file name.

Reduce length of TAP test file name.

Buildfarm member fairywren hit the Windows limitation on the length of a
file path. While there may be other things we should also do to prevent
this from happening, it's certainly the case that the length of this
test file name is much longer than others in the same directory, so make
it shorter.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: http://postgr.es/m/274e0a1a-d7d2-4bc8-8b56-dd09f285715e@gmail.com
Backpatch-through: 17