test: Drop x modifier in fsbridge::fopen call for mingw builds

The MinGW-w64 toolchain links executables to the old msvcrt C Runtime
Library that does not support the `x` modifier for the _wfopen()
function.

src/test/streams_tests.cpp

@@ -29,7 +29,14 @@ BOOST_AUTO_TEST_CASE(xor_file)
         BOOST_CHECK_EXCEPTION(xor_file.ignore(1), std::ios_base::failure, HasReason{"AutoFile::ignore: file handle is nullpt"});
     }
     {
-        AutoFile xor_file{raw_file("wbx"), xor_pat};
+#ifdef __MINGW64__
+        // Our usage of mingw-w64 and the msvcrt runtime does not support
+        // the x modifier for the _wfopen().
+        const char* mode = "wb";
+#else
+        const char* mode = "wbx";
+#endif
+        AutoFile xor_file{raw_file(mode), xor_pat};
         xor_file << test1 << test2;
     }
     {