postgres_fdw: Split out the query_cancel test to its own file

postgres_fdw: Split out the query_cancel test to its own file

This allows us to skip it in Cygwin, where it's reportedly flaky because
of platform bugs or something.

Backpatch to 17, where the test was introduced by commit 2466d6654f85.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/e4d0cb33-6be5-e4d5-ae49-9eac3ff2b005@gmail.com