types/logger: add TestLogger

We have this in another repo and I wanted it here too.

Updates #cleanup

Change-Id: If93dc73f11eaaada5024acf2a885a153b88db5a0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/logger/logger.go

@@ -373,3 +373,19 @@ func (a asJSONResult) Format(s fmt.State, verb rune) {
 	}
 	s.Write(v)
 }
+
+// TBLogger is the testing.TB subset needed by TestLogger.
+type TBLogger interface {
+	Helper()
+	Logf(format string, args ...any)
+}
+
+// TestLogger returns a logger that logs to tb.Logf
+// with a prefix to make it easier to distinguish spam
+// from explicit test failures.
+func TestLogger(tb TBLogger) Logf {
+	return func(format string, args ...any) {
+		tb.Helper()
+		tb.Logf("    ... "+format, args...)
+	}
+}