cmd/testwrapper/flakytest: skip flaky tests if TS_SKIP_FLAKY_TESTS set

This is for a future test scheduler, so it can run potentially flaky
tests separately, doing all the non-flaky ones together in one batch.

Updates tailscale/corp#28679

Change-Id: Ic4a11f9bf394528ef75792fd622f17bc01a4ec8a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/testwrapper/flakytest/flakytest.go

@@ -11,6 +11,7 @@
 	"os"
 	"path"
 	"regexp"
+	"strconv"
 	"sync"
 	"testing"
 
@@ -60,6 +61,10 @@ func Mark(t testing.TB, issue string) {
 	// And then remove this Logf a month or so after that.
 	t.Logf("flakytest: issue tracking this flaky test: %s", issue)
 
+	if boolEnv("TS_SKIP_FLAKY_TESTS") {
+		t.Skipf("skipping due to TS_SKIP_FLAKY_TESTS")
+	}
+
 	// Record the root test name as flakey.
 	rootFlakesMu.Lock()
 	defer rootFlakesMu.Unlock()
@@ -80,3 +85,12 @@ func Marked(t testing.TB) bool {
 	}
 	return false
 }
+
+func boolEnv(k string) bool {
+	s := os.Getenv(k)
+	if s == "" {
+		return false
+	}
+	v, _ := strconv.ParseBool(s)
+	return v
+}