More fixes for https://github.com/cppformat/cppformat/issues/50.

test/gtest-extra.h

@@ -81,11 +81,11 @@
   FMT_TEST_THROW_(statement, expected_exception, \
       expected_message, GTEST_NONFATAL_FAILURE_)
 
-std::string FormatSystemErrorMessage(int error_code, fmt::StringRef message);
+std::string format_system_error(int error_code, fmt::StringRef message);
 
 #define EXPECT_SYSTEM_ERROR(statement, error_code, message) \
   EXPECT_THROW_MSG(statement, fmt::SystemError, \
-      FormatSystemErrorMessage(error_code, message))
+      format_system_error(error_code, message))
 
 #if FMT_USE_FILE_DESCRIPTORS