sql/logictest: fix flakes in select_for_update_read_committed

The `select_for_update_read_committed` tests were flaking because not
all statements were being run under READ COMMITTED isolation. The logic
test infrastructure does not allow fine-grained control of sessions, and
setting the isolation level in one statement would only apply to a
single session. Subsequent statements are not guaranteed to run in the
same session because they could run in any session in the connection
pool. This commit wraps each statement in an explicitly transaction with
an explicit isolation level to ensure READ COMMITTED is used.

In the future, we should investigate allowing fine-grained and explicit
control of sessions in logic tests.

Fixes #112677

Release note: None

pkg/sql/logictest/testdata/logic_test/select_for_update_read_committed

@@ -3,9 +3,6 @@
 statement ok
 SET CLUSTER SETTING sql.txn.read_committed_syntax.enabled = true
 
-statement ok
-SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED
-
 statement ok
 CREATE TABLE abc (a INT PRIMARY KEY, b INT, c INT, INDEX (b), FAMILY (a, b, c))
 
@@ -29,7 +26,7 @@ GRANT ALL on bcd TO testuser
 user testuser
 
 statement ok
-BEGIN
+BEGIN ISOLATION LEVEL READ COMMITTED
 
 query III rowsort
 SELECT * FROM abc WHERE a != 3 FOR UPDATE
@@ -50,14 +47,18 @@ user root
 # Normal reads do not block.
 
 query III rowsort
-SELECT * FROM abc
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM abc;
+COMMIT;
 ----
 1  10  100
 2  20  200
 3  30  300
 
 query III rowsort
-SELECT * FROM bcd
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM bcd;
+COMMIT;
 ----
 20  200  2000
 30  300  3000
@@ -66,28 +67,34 @@ SELECT * FROM bcd
 # SKIP LOCKED reads do not block.
 
 query III rowsort
-SELECT * FROM abc FOR UPDATE SKIP LOCKED
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM abc FOR UPDATE SKIP LOCKED;
+COMMIT;
 ----
 3  30  300
 
 query III rowsort
-SELECT * FROM bcd FOR UPDATE SKIP LOCKED
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM bcd FOR UPDATE SKIP LOCKED;
+COMMIT;
 ----
 20  200  2000
-30  300  3000
-40  400  4000
 
 # Shared reads block on exclusive locks but not on shared locks.
 
 query III async,rowsort q00
-SELECT * FROM abc FOR SHARE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM abc FOR SHARE;
+COMMIT;
 ----
 1  11  101
 2  21  201
 3  30  300
 
 query III rowsort
-SELECT * FROM bcd FOR SHARE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM bcd FOR SHARE;
+COMMIT;
 ----
 20  200  2000
 30  300  3000
@@ -96,14 +103,18 @@ SELECT * FROM bcd FOR SHARE
 # Exclusive reads block on both.
 
 query III async,rowsort q01
-SELECT * FROM abc FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM abc FOR UPDATE;
+COMMIT;
 ----
 1  11  101
 2  21  201
 3  30  300
 
 query III async,rowsort q02
-SELECT * FROM bcd FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM bcd FOR UPDATE;
+COMMIT;
 ----
 20  200  2000
 30  300  3000
@@ -112,42 +123,56 @@ SELECT * FROM bcd FOR UPDATE
 # Try more exclusive-locking queries.
 
 query I async q03
-SELECT a FROM abc WHERE a = 2 FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT a FROM abc WHERE a = 2 FOR UPDATE;
+COMMIT;
 ----
 2
 
 query I async q04
-SELECT b FROM abc WHERE a = 2 FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT b FROM abc WHERE a = 2 FOR UPDATE;
+COMMIT;
 ----
 21
 
 query I async q05
-SELECT c FROM abc WHERE a = 2 FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT c FROM abc WHERE a = 2 FOR UPDATE;
+COMMIT;
 ----
 201
 
 query I async q06
-SELECT c FROM abc ORDER BY a DESC LIMIT 2 FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT c FROM abc ORDER BY a DESC LIMIT 2 FOR UPDATE;
+COMMIT;
 ----
 300
 201
 
 query I async,rowsort q07
-SELECT a + b + c FROM abc FOR UPDATE
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT a + b + c FROM abc FOR UPDATE;
+COMMIT;
 ----
 113
 224
 333
 
 # Try some joins
 
-query IIIII async q08
-SELECT * FROM abc JOIN bcd USING (b) FOR SHARE
+query IIIII async,rowsort q08
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM abc JOIN bcd USING (b) FOR SHARE;
+COMMIT;
 ----
 30  3  300  300  3000
 
-query IIIII async q09
-SELECT * FROM abc JOIN bcd USING (c) FOR UPDATE
+query IIIII async,rowsort q09
+BEGIN ISOLATION LEVEL READ COMMITTED;
+SELECT * FROM abc JOIN bcd USING (c) FOR UPDATE;
+COMMIT;
 ----
 300  3  30  30  3000