Add test for formatting large (u)int64 numbers

As reported in #235 formatting the first number which doesn't fit into
int64_t anymore fails to add the thousands separators.
I.e.:
`9223372036854775807` -> `9,223,372,036,854,775,807`
`9223372036854775808` -> `9223372036854775808`

Add a test reproducing that that for all backends.

test/formatting_common.hpp

@@ -92,4 +92,40 @@ void test_parse_multi_number()
     BOOST_LOCALE_CALL(wchar_t);
 #undef BOOST_LOCALE_CALL
 #undef BOOST_LOCALE_CALL_I
-}
+}
+
+template<typename CharType>
+void test_format_large_number_by_char(const std::locale& locale)
+{
+    std::basic_ostringstream<CharType> output;
+    output.imbue(locale);
+    output << boost::locale::as::number;
+
+    constexpr int64_t high_signed64 = 9223372036854775807;
+    static_assert(high_signed64 == std::numeric_limits<int64_t>::max(), "Value must match");
+
+    empty_stream(output) << high_signed64;
+    TEST_EQ(output.str(), ascii_to<CharType>("9,223,372,036,854,775,807"));
+    empty_stream(output) << static_cast<uint64_t>(high_signed64);
+    TEST_EQ(output.str(), ascii_to<CharType>("9,223,372,036,854,775,807"));
+    empty_stream(output) << (static_cast<uint64_t>(high_signed64) + 1u);
+    TEST_EQ(output.str(), ascii_to<CharType>("9,223,372,036,854,775,808"));
+    empty_stream(output) << (static_cast<uint64_t>(high_signed64) + 579u);
+    TEST_EQ(output.str(), ascii_to<CharType>("9,223,372,036,854,776,386"));
+}
+
+void test_format_large_number()
+{
+    const auto locale = boost::locale::generator{}("en_US.UTF-8");
+
+    std::cout << "Testing char" << std::endl;
+    test_format_large_number_by_char<char>(locale);
+
+    std::cout << "Testing wchar_t" << std::endl;
+    test_format_large_number_by_char<wchar_t>(locale);
+
+#ifdef BOOST_LOCALE_ENABLE_CHAR16_T
+    std::cout << "Testing char16_t" << std::endl;
+    test_format_large_number_by_char<char16_t>(locale);
+#endif
+}

test/test_formatting.cpp

@@ -891,6 +891,7 @@ void test_main(int argc, char** argv)
     test_format_class<char32_t>();
 #endif
 
+    test_format_large_number();
     test_parse_multi_number();
 }
 

test/test_posix_formatting.cpp

@@ -186,6 +186,7 @@ void test_main(int /*argc*/, char** /*argv*/)
             TEST(v == "12345,45" || v == "12 345,45" || v == "12.345,45");
         }
     }
+    test_format_large_number();
     test_parse_multi_number();
 }
 

test/test_std_formatting.cpp

@@ -233,8 +233,10 @@ void test_main(int /*argc*/, char** /*argv*/)
     }
     // Std backend silently falls back to the C locale when the locale is not supported
     // which breaks the test assumptions
-    if(has_std_locale("en_US.UTF-8"))
+    if(has_std_locale("en_US.UTF-8")) {
+        test_format_large_number();
         test_parse_multi_number();
+    }
 }
 
 // boostinspect:noascii

test/test_winapi_formatting.cpp

@@ -177,6 +177,7 @@ void test_main(int /*argc*/, char** /*argv*/)
             test_by_char<wchar_t>(l, name, name_lcid.second);
         }
     }
+    test_format_large_number();
     test_parse_multi_number();
     std::cout << "- Testing strftime" << std::endl;
     test_date_time(gen("en_US.UTF-8"));