Move isolation test index-killtuples to src/test/modules/index/

Move isolation test index-killtuples to src/test/modules/index/

index-killtuples test depends on the contrib modules btree_gin and
btree_gist, which would not be installed in a temporary installation
with an execution of the main isolation test suite like this one:
make -C src/test/isolation/ check

src/test/isolation/ should not depend on contrib/, and EXTRA_INSTALL has
no effect in this case as this test suite uses its own Makefile rules.

This commit moves index-killtuples into its new module, called "index",
whose name looks like the best fit there can be as it depends on more
than one index AM.  btree_gin and btree_gist are now pulled in the
temporary installation with EXTRA_INSTALL.  The test is renamed to
"killtuples", for simplicity.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Suggested-by: Andres Freund <andres@anarazel.de>
Suggested-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aKJsWedftW7UX1WM@paquier.xyz