Use override consistently in leveldb::test::ErrorEnv.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=239453565

util/testutil.h

@@ -40,8 +40,8 @@ class ErrorEnv : public EnvWrapper {
     delete target();
   }
 
-  virtual Status NewWritableFile(const std::string& fname,
-                                 WritableFile** result) {
+  Status NewWritableFile(const std::string& fname,
+                         WritableFile** result) override {
     if (writable_file_error_) {
       ++num_writable_file_errors_;
       *result = nullptr;
@@ -50,8 +50,8 @@ class ErrorEnv : public EnvWrapper {
     return target()->NewWritableFile(fname, result);
   }
 
-  virtual Status NewAppendableFile(const std::string& fname,
-                                   WritableFile** result) {
+  Status NewAppendableFile(const std::string& fname,
+                           WritableFile** result) override {
     if (writable_file_error_) {
       ++num_writable_file_errors_;
       *result = nullptr;