Try not to write files in tests.

libdevcrypto/SecretStore.cpp

@@ -88,6 +88,7 @@ static js::mValue upgraded(string const& _s)
 
 SecretStore::SecretStore(string const& _path): m_path(_path)
 {
+	if (!m_path.empty())
 		load();
 }
 
@@ -137,6 +138,9 @@ void SecretStore::clearCache() const
 
 void SecretStore::save(string const& _keysPath)
 {
+	if (_keysPath.empty())
+		return;
+
 	fs::path p(_keysPath);
 	fs::create_directories(p);
 	for (auto& k: m_keys)