schemachanger: don't run TestAlterTableDMLInjection with t.Parallel

Under deadlock/race configurations, this can add too much load to the
test runner.

Release note: None

Author: rafiss

pkg/sql/schemachanger/dml_injection_test.go

@@ -563,7 +563,6 @@ func TestAlterTableDMLInjection(t *testing.T) {
 	ctx := context.Background()
 	for _, tc := range testCases {
 		t.Run(tc.desc, tc.capture(func(t *testing.T, tc testCase) {
-			t.Parallel() // SAFE FOR TESTING
 			if issue := tc.skipIssue; issue != 0 {
 				skip.WithIssue(t, issue)
 			}