Enable routine running of citext's UTF8-specific test cases.

Enable routine running of citext's UTF8-specific test cases.

These test cases have been commented out since citext was invented,
because at the time we had no nice way to deal with tests that
have restrictions such as requiring UTF8 encoding.  But now we do
have a convention for that, ie put them into a separate test file
with an early-exit path.  So let's enable these tests to run when
their prerequisites are satisfied.

(We may have to tighten the prerequisites beyond the "encoding = UTF8
and locale != C" checks made here.  But let's put it on the buildfarm
and see what blows up.)

Dag Lem

Discussion: https://postgr.es/m/ygezgoacs4e.fsf_-_@sid.nimrod.no