Add tests.

test/gtest-extra-test.cc

@@ -348,6 +348,12 @@ TEST(StreamingAssertionsTest, EXPECT_WRITE) {
       << "expected failure", "expected failure");
 }
 
+TEST(UtilTest, FormatSystemErrorMessage) {
+  fmt::Writer out;
+  fmt::internal::FormatSystemErrorMessage(out, EDOM, "test message");
+  EXPECT_EQ(out.str(), FormatSystemErrorMessage(EDOM, "test message"));
+}
+
 #if FMT_USE_FILE_DESCRIPTORS
 
 // Checks if the file is open by reading one character from it.
@@ -800,8 +806,6 @@ TEST(OutputRedirectTest, ErrorInDtor) {
   write_copy.dup2(write_fd); // "undo" close or dtor of BufferedFile will fail
 }
 
-// TODO: test retry on EINTR, test FormatSystemErrorMessage
-
 #endif  // FMT_USE_FILE_DESCRIPTORS
 
 }  // namespace