runner: prepend the resource directory for async-signing tests.

b4d65fda70db407a560e12cdba60a87c0ef32937 was written concurrently with
my updating runner to handle -resource-dir (in
7c803a65d5c52e56b4eca30ec48ad52b90e0e36c) and thus it didn't include the
needed change for the test that it added to handle it.

This change fixes that added test so that it can run with -resource-dir.

Change-Id: I06b0adfb3fcf3f11c061fe1c8332a45cd7cd2dbc
diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 1186313..2e2c132 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -2210,8 +2210,8 @@
 				ClientAuth: RequireAnyClientCert,
 			flags: []string{
-				"-cert-file", rsaCertificateFile,
-				"-key-file", rsaKeyFile,
+				"-cert-file", path.Join(*resourceDir, rsaCertificateFile),
+				"-key-file", path.Join(*resourceDir, rsaKeyFile),