lint: add a linter to forbid uses of t.Parallel()

This suggests using `sync.WaitGroup` instead.

Release note: None

Author: knz

pkg/sql/logictest/logic.go

@@ -2145,7 +2145,7 @@ func RunLogicTest(t *testing.T, globs ...string) {
 						// the batch size is a global variable.
 						// TODO(jordan, radu): make sqlbase.kvBatchSize non-global to fix this.
 						if filepath.Base(path) != "select_index_span_ranges" {
-							t.Parallel()
+							t.Parallel() // SAFE FOR TESTING (this comments satisfies the linter)
 						}
 					}
 					lt := logicTest{

pkg/testutils/lint/lint_test.go

@@ -748,6 +748,42 @@ func TestLint(t *testing.T) {
 		}
 	})
 
+	t.Run("TestTParallel", func(t *testing.T) {
+		t.Parallel()
+		cmd, stderr, filter, err := dirCmd(
+			pkgDir,
+			"git",
+			"grep",
+			"-nE",
+			`\.Parallel\(\)`,
+			"--",
+			"*.go",
+			":!testutils/lint/*.go",
+		)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if err := cmd.Start(); err != nil {
+			t.Fatal(err)
+		}
+
+		if err := stream.ForEach(stream.Sequence(
+			filter,
+			stream.GrepNot(`// SAFE FOR TESTING`),
+		), func(s string) {
+			t.Errorf("\n%s <- forbidden, use a sync.WaitGroup instead (cf https://github.com/golang/go/issues/31651)", s)
+		}); err != nil {
+			t.Error(err)
+		}
+
+		if err := cmd.Wait(); err != nil {
+			if out := stderr.String(); len(out) > 0 {
+				t.Fatalf("err=%s, stderr=%s", err, out)
+			}
+		}
+	})
+
 	t.Run("TestProtoMarshal", func(t *testing.T) {
 		t.Parallel()
 		cmd, stderr, filter, err := dirCmd(

pkg/util/fast_int_map_test.go

@@ -33,7 +33,7 @@ func TestFastIntMap(t *testing.T) {
 	}
 	for _, tc := range cases {
 		t.Run(fmt.Sprintf("%d-%d", tc.keyRange, tc.valRange), func(t *testing.T) {
-			t.Parallel()
+			t.Parallel() // SAFE FOR TESTING (this comment is for the linter)
 			rng, _ := randutil.NewPseudoRand()
 			var fm FastIntMap
 			m := make(map[int]int)

pkg/util/fast_int_set_test.go

@@ -26,7 +26,7 @@ func TestFastIntSet(t *testing.T) {
 	for _, mVal := range []int{1, 8, 30, smallCutoff, 2 * smallCutoff, 4 * smallCutoff} {
 		m := mVal
 		t.Run(fmt.Sprintf("%d", m), func(t *testing.T) {
-			t.Parallel()
+			t.Parallel() // SAFE FOR TESTING (this comment is for the linter)
 			rng, _ := randutil.NewPseudoRand()
 			in := make([]bool, m)
 			forEachRes := make([]bool, m)