Attempt to fix test interference from foreign_key opfamily test

The test added by c62b330912 creates a btree opfamily whose members
include built-in integer operators, which get_mergejoin_opfamilies()
finds regardless of schema.  So any concurrently running test that
planned an integer join or sort could pick the family up while it
existed.  That's the likely cause of the intermittent plan-shape
changes seen in window.sql and equivclass.sql on the buildfarm after
that commit.

Run the test inside a transaction that is rolled back, so the family
is never committed and never visible to other backends.

Per buildfarm members sifaka, prion, turaca, widowbird.

Discussion: https://postgr.es/m/CA+HiwqGitHCmO7nvV4nXXYWDkNYA+zVcJj=JNmL3C1O9B0Gz0A@mail.gmail.com
Backpatch-though: 19

src/test/regress/expected/foreign_key.out

@@ -1084,7 +1084,12 @@ DETAIL:  Key columns "ptest4" of the referencing table and "ptest1" of the refer
 -- Replace the equality operator the FK recorded with an identical
 -- implementation, so only opfamily membership changes.  The recorded operator
 -- is now absent from the family; the fast path must fall back to SPI instead
--- of probing with it.
+-- of probing with it.  Run inside a transaction that is rolled back: the
+-- family holds built-in integer operators, and the planner finds btree
+-- opfamilies by content (get_mergejoin_opfamilies), not by schema, so if it
+-- were committed it would be visible to concurrent tests and disturb their
+-- plans.
+begin;
 create schema fk_opfamily;
 set search_path = fk_opfamily, pg_catalog;
 create operator family fam using btree;
@@ -1116,19 +1121,21 @@ insert into p values (1), (2);
 insert into warm values (1);
 -- Change only pg_amop.  warm's cached metadata now names an operator the
 -- opfamily no longer contains; cold is still evaluated fresh.
-begin;
 alter operator family fam using btree drop operator 3(integer,bigint);
 alter operator family fam using btree add operator 3 =#=(integer,bigint);
-commit;
 -- A present key must be accepted and a missing one rejected, via SPI.
 insert into warm values (2);
+savepoint s;
 insert into warm values (99);
 ERROR:  insert or update on table "warm" violates foreign key constraint "warm_k_fkey"
 DETAIL:  Key (k)=(99) is not present in table "p".
+rollback to s;
 insert into cold values (2);
+savepoint s;
 insert into cold values (99);
 ERROR:  insert or update on table "cold" violates foreign key constraint "cold_k_fkey"
 DETAIL:  Key (k)=(99) is not present in table "p".
+rollback to s;
 select * from warm order by k;
  k 
 ---
@@ -1143,11 +1150,7 @@ select * from cold order by k;
 (1 row)
 
 reset search_path;
-drop table fk_opfamily.warm, fk_opfamily.cold, fk_opfamily.p;
-drop operator class fk_opfamily.int_ops using btree;
-drop operator family fk_opfamily.fam using btree;
-drop operator fk_opfamily.=#=(integer,bigint);
-drop schema fk_opfamily;
+rollback;
 --
 -- Now some cases with inheritance
 -- Basic 2 table case: 1 column of matching types.

src/test/regress/sql/foreign_key.sql

@@ -750,7 +750,12 @@ ptest3) REFERENCES pktable);
 -- Replace the equality operator the FK recorded with an identical
 -- implementation, so only opfamily membership changes.  The recorded operator
 -- is now absent from the family; the fast path must fall back to SPI instead
--- of probing with it.
+-- of probing with it.  Run inside a transaction that is rolled back: the
+-- family holds built-in integer operators, and the planner finds btree
+-- opfamilies by content (get_mergejoin_opfamilies), not by schema, so if it
+-- were committed it would be visible to concurrent tests and disturb their
+-- plans.
+begin;
 create schema fk_opfamily;
 set search_path = fk_opfamily, pg_catalog;
 create operator family fam using btree;
@@ -784,24 +789,22 @@ insert into warm values (1);
 
 -- Change only pg_amop.  warm's cached metadata now names an operator the
 -- opfamily no longer contains; cold is still evaluated fresh.
-begin;
 alter operator family fam using btree drop operator 3(integer,bigint);
 alter operator family fam using btree add operator 3 =#=(integer,bigint);
-commit;
 
 -- A present key must be accepted and a missing one rejected, via SPI.
 insert into warm values (2);
+savepoint s;
 insert into warm values (99);
+rollback to s;
 insert into cold values (2);
+savepoint s;
 insert into cold values (99);
+rollback to s;
 select * from warm order by k;
 select * from cold order by k;
 reset search_path;
-drop table fk_opfamily.warm, fk_opfamily.cold, fk_opfamily.p;
-drop operator class fk_opfamily.int_ops using btree;
-drop operator family fk_opfamily.fam using btree;
-drop operator fk_opfamily.=#=(integer,bigint);
-drop schema fk_opfamily;
+rollback;
 
 --
 -- Now some cases with inheritance