test: fix _mm_log() prototypes

GCC 13 is unhappy to cast a MMLogLevel to a guint32:

  [188/539] Compiling C object src/plugins/test-shared-icera.p/icera_tests_test-modem-helpers-icera.c.o
  In file included from ../src/plugins/icera/tests/test-modem-helpers-icera.c:27:
  ../src/mm-log-test.h:25:1: warning: conflicting types for ‘_mm_log’ due to enum/integer mismatch; have ‘void(void *, const gchar *, const gchar *, const gchar *, guint32,  const gchar *, ...)’ {aka ‘void(void *, const char *, const char *, const char *, unsigned int,  const char *, ...)’} [-Wenum-int-mismatch]
     25 | _mm_log (gpointer     obj,
        | ^~~~~~~
  In file included from ../src/mm-log-test.h:20:
  ../src/mm-log.h:61:6: note: previous declaration of ‘_mm_log’ with type ‘void(void *, const gchar *, const gchar *, const gchar *, MMLogLevel,  const gchar *, ...)’ {aka ‘void(void *, const char *, const char *, const char *, MMLogLevel,  const char *, ...)’}
     61 | void _mm_log (gpointer     obj,
        |      ^~~~~~~

That's perfectly fine, just use the enum type directly.

test/mmtty.c

@@ -269,7 +269,7 @@ _mm_log (gpointer     obj,
          const gchar *module,
          const gchar *loc,
          const gchar *func,
-         guint32      level,
+         MMLogLevel   level,
          const gchar *fmt,
          ...)
 {