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 2466d6654f.

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

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2760,22 +2760,6 @@ SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c
 (10 rows)
 
 ALTER VIEW v4 OWNER TO regress_view_owner;
--- Make sure this big CROSS JOIN query is pushed down
-EXPLAIN (VERBOSE, COSTS OFF) SELECT count(*) FROM ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5;
-                                                                             QUERY PLAN                                                                              
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
- Foreign Scan
-   Output: (count(*))
-   Relations: Aggregate on ((((public.ft1) INNER JOIN (public.ft2)) INNER JOIN (public.ft4)) INNER JOIN (public.ft5))
-   Remote SQL: SELECT count(*) FROM ((("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (TRUE)) INNER JOIN "S 1"."T 3" r4 ON (TRUE)) INNER JOIN "S 1"."T 4" r6 ON (TRUE))
-(4 rows)
-
--- Make sure query cancellation works
-BEGIN;
-SET LOCAL statement_timeout = '10ms';
-select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5; -- this takes very long
-ERROR:  canceling statement due to statement timeout
-COMMIT;
 -- ====================================================================
 -- Check that userid to use when querying the remote table is correctly
 -- propagated into foreign rels present in subqueries under an UNION ALL