Make experimental capture work on Windows with read-write temp file behavior

include/internal/catch_output_redirect.cpp

@@ -66,7 +66,7 @@ namespace Catch {
         if (tmpnam_s(m_buffer)) {
             CATCH_RUNTIME_ERROR("Could not get a temp filename");
         }
-        if (fopen_s(&m_file, m_buffer, "w")) {
+        if (fopen_s(&m_file, m_buffer, "w+")) {
             char buffer[100];
             if (strerror_s(buffer, errno)) {
                 CATCH_RUNTIME_ERROR("Could not translate errno to a string");