Fix signedness warning in tests/mainloop-test.c

tests/mainloop-test.c: In function ‘cleanup_crawlers’:
tests/mainloop-test.c:358:15: error: comparison of integer expressions of different signedness: ‘gint’ {aka ‘int’} and ‘guint’ {aka ‘unsigned int’}
  358 |   for (i=0; i < crawler_array->len; i++)
      |               ^
diff --git a/tests/mainloop-test.c b/tests/mainloop-test.c
index 5a3e6c9..be6024a 100644
--- a/tests/mainloop-test.c
+++ b/tests/mainloop-test.c
@@ -352,10 +352,10 @@
 static void
 cleanup_crawlers (GMainContext *context)
 {
-  gint i;
+  guint i;
   
   G_LOCK (crawler_array_lock);
-  for (i=0; i < crawler_array->len; i++)
+  for (i = 0; i < crawler_array->len; i++)
     {
       if (g_source_get_context (crawler_array->pdata[i]) == context)
 	{