Now that we have TAP tests, a contrib module may have something useful
to do in "make check" even if it has no pg_regress-style regression
scripts, and hence no REGRESS setting.  But the TAP tests will fail,
or else test the wrong installed files, unless we install the contrib
module into the temp installation.  So move the bit about adding to
EXTRA_INSTALL so that it applies regardless.

We might want this in back branches in future, but for the moment
I only risked adding it to v11.

Discussion: https://postgr.es/m/12438.1535488750@sss.pgh.pa.us