internal/trace: run children in race mode during race testing

This would have caught golang#65607.

On my machine:

internal/trace/v2:               ~13s
internal/trace/v2: -short:        ~4s
internal/trace/v2: -race -short: ~20s
internal/trace/v2: -race:        ~46s

For golang#65607.

Change-Id: I03a555ebaf8c761c98a82b6d35fa16d7a99aa9e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/562998
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/internal/trace/v2/trace_test.go

@@ -8,6 +8,7 @@ import (
 	"bufio"
 	"bytes"
 	"fmt"
+	"internal/race"
 	"internal/testenv"
 	"internal/trace/v2"
 	"internal/trace/v2/testtrace"
@@ -541,7 +542,11 @@ func testTraceProg(t *testing.T, progName string, extra func(t *testing.T, trace
 	testName := progName
 	runTest := func(t *testing.T, stress bool) {
 		// Run the program and capture the trace, which is always written to stdout.
-		cmd := testenv.Command(t, testenv.GoToolPath(t), "run", testPath)
+		cmd := testenv.Command(t, testenv.GoToolPath(t), "run")
+		if race.Enabled {
+			cmd.Args = append(cmd.Args, "-race")
+		}
+		cmd.Args = append(cmd.Args, testPath)
 		cmd.Env = append(os.Environ(), "GOEXPERIMENT=exectracer2")
 		if stress {
 			// Advance a generation constantly.