Fix typo (forgotten reference operator) in test.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171708408

db/fault_injection_test.cc

@@ -34,7 +34,7 @@ class FaultInjectionTestEnv;
 namespace {
 
 // Assume a filename, and not a directory name like "/foo/bar/"
-static std::string GetDirName(const std::string filename) {
+static std::string GetDirName(const std::string& filename) {
   size_t found = filename.find_last_of("/\\");
   if (found == std::string::npos) {
     return "";