kv: deflake TestPriorityRatchetOnAbortOrPush

Fixes #68584.

The test was flaky for the reasons described in #68584. There doesn't appear to
be an easy way to fix this behavior, and it's not clear how valuable doing so
even is given how little we rely on transaction priorities anymore, so the
commit just deflakes the test by rejecting them.

Release justification: deflaking a test.

pkg/kv/kvclient/kvcoord/txn_test.go

@@ -202,7 +202,17 @@ func TestLostUpdate(t *testing.T) {
 func TestPriorityRatchetOnAbortOrPush(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
-	s := createTestDB(t)
+	s := createTestDBWithKnobs(t, &kvserver.StoreTestingKnobs{
+		TestingRequestFilter: func(_ context.Context, ba roachpb.BatchRequest) *roachpb.Error {
+			// Reject transaction heartbeats, which can make the test flaky when they
+			// detect an aborted transaction before the Get operation does. See #68584
+			// for an explanation.
+			if ba.IsSingleHeartbeatTxnRequest() {
+				return roachpb.NewErrorf("rejected")
+			}
+			return nil
+		},
+	})
 	defer s.Stop()
 
 	pushByReading := func(key roachpb.Key) {