Fix transaction.sql tests in higher isolation levels.

It seems like a useful sanity check to be able to run "installcheck"
against a cluster running with default_transaction_level set to
serializable or repeatable read.  Only one thing currently fails in
those configurations, so let's fix that.

No back-patch for now, because it fails in many other places in some of
the stable branches.  We'd have to go back and fix those too if we
included this configuration in automated testing.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGJUaHeK%3DHLATxF1JOKDjKJVrBKA-zmbPAebOM0Se2FQRg%40mail.gmail.com

src/test/regress/expected/transactions.out

@@ -1026,6 +1026,7 @@ SHOW transaction_isolation;  -- transaction is active at this point
 (1 row)
 
 ROLLBACK;
+SET default_transaction_isolation = 'read committed';
 -- START TRANSACTION + COMMIT/ROLLBACK + COMMIT/ROLLBACK AND CHAIN
 START TRANSACTION ISOLATION LEVEL REPEATABLE READ\; INSERT INTO abc VALUES (17)\; COMMIT\; INSERT INTO abc VALUES (18)\; COMMIT AND CHAIN;  -- 17 commit, 18 error
 ERROR:  COMMIT AND CHAIN can only be used in transaction blocks
@@ -1043,6 +1044,7 @@ SHOW transaction_isolation;  -- out of transaction block
  read committed
 (1 row)
 
+RESET default_transaction_isolation;
 SELECT * FROM abc ORDER BY 1;
  a  
 ----

src/test/regress/sql/transactions.sql

@@ -578,6 +578,8 @@ START TRANSACTION ISOLATION LEVEL REPEATABLE READ\; INSERT INTO abc VALUES (16)\
 SHOW transaction_isolation;  -- transaction is active at this point
 ROLLBACK;
 
+SET default_transaction_isolation = 'read committed';
+
 -- START TRANSACTION + COMMIT/ROLLBACK + COMMIT/ROLLBACK AND CHAIN
 START TRANSACTION ISOLATION LEVEL REPEATABLE READ\; INSERT INTO abc VALUES (17)\; COMMIT\; INSERT INTO abc VALUES (18)\; COMMIT AND CHAIN;  -- 17 commit, 18 error
 SHOW transaction_isolation;  -- out of transaction block
@@ -585,6 +587,8 @@ SHOW transaction_isolation;  -- out of transaction block
 START TRANSACTION ISOLATION LEVEL REPEATABLE READ\; INSERT INTO abc VALUES (19)\; ROLLBACK\; INSERT INTO abc VALUES (20)\; ROLLBACK AND CHAIN;  -- 19 rollback, 20 error
 SHOW transaction_isolation;  -- out of transaction block
 
+RESET default_transaction_isolation;
+
 SELECT * FROM abc ORDER BY 1;
 
 DROP TABLE abc;