cmd/testwrapper: only retry flaky failed tests

Redo the testwrapper to track and only retry flaky tests instead
of retrying the entire pkg. It also fails early if a non-flaky test fails.

This also makes it so that the go test caches are used.

Fixes #7975

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/testwrapper/flakytest/flakytest_test.go

@@ -3,7 +3,10 @@
 
 package flakytest
 
-import "testing"
+import (
+	"os"
+	"testing"
+)
 
 func TestIssueFormat(t *testing.T) {
 	testCases := []struct {
@@ -24,3 +27,14 @@ func TestIssueFormat(t *testing.T) {
 		}
 	}
 }
+
+func TestFlakeRun(t *testing.T) {
+	Mark(t, "https://github.com/tailscale/tailscale/issues/0") // random issue
+	e := os.Getenv(FlakeAttemptEnv)
+	if e == "" {
+		t.Skip("not running in testwrapper")
+	}
+	if e == "1" {
+		t.Fatal("failing on purpose")
+	}
+}