tests: Fix a compiler warning in tests -fpermissive

Prusa-Firmware/tests/../Firmware/util.h:114:25: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
  114 |         pStrEnd = strchr(this->ptr, '"');
      |                   ~~~~~~^~~~~~~~~~~~~~~~
      |                         |
      |                         const char*

Firmware/util.h

@@ -96,7 +96,7 @@ public:
     : len(0)
     , found(false)
     {
-        char * pStrEnd = NULL;
+        const char * pStrEnd = NULL;
 
         // Start of the string
         this->ptr = strchr(pStr, '"');