fix: fix unfinished u8 fopen.

- finish unfinished utf8 version fopen.
- add testbench for it.

testbench/yycc/patch/fopen.cpp

@@ -0,0 +1,24 @@
+#include <gtest/gtest.h>
+#include <yycc.hpp>
+#include <yycc/patch/fopen.hpp>
+
+#define FOPEN ::yycc::patch::fopen
+
+namespace yycctest::patch::fopen {
+
+    TEST(PatchFopen, Normal) {
+        FILE* handle;
+
+#if defined(YYCC_OS_WINDOWS)
+        // In Windows, we can always visit NUL device.
+        handle = FOPEN::fopen(u8"NUL", u8"wb");
+#else
+        // In other system following UNIX design, we can visit /dev/null device.
+        handle = FOPEN::fopen(u8"/dev/null", u8"wb");
+#endif
+
+        ASSERT_TRUE(handle != nullptr);
+        std::fclose(handle);
+    }
+
+} // namespace yycctest::patch::fopen