Fix signedness warning in tests/dirname-test.c

tests/dirname-test.c: In function ‘main’:
tests/dirname-test.c:105:17: error: comparison of integer expressions of different signedness: ‘gint’ {aka ‘int’} and ‘guint’ {aka ‘unsigned int’}
  105 |   for (i = 0; i < n_dirname_checks; i++)
      |                 ^
diff --git a/tests/dirname-test.c b/tests/dirname-test.c
index 52a4678..81c55d5 100644
--- a/tests/dirname-test.c
+++ b/tests/dirname-test.c
@@ -53,7 +53,7 @@
 main (int   argc,
       char *argv[])
 {
-  gint i;
+  gsize i;
   struct {
     gchar *filename;
     gchar *dirname;
@@ -100,9 +100,8 @@
     { "a:\\/", "a:\\" },
 #endif
   };
-  guint n_dirname_checks = sizeof (dirname_checks) / sizeof (dirname_checks[0]);
 
-  for (i = 0; i < n_dirname_checks; i++)
+  for (i = 0; i < G_N_ELEMENTS (dirname_checks); i++)
     {
       gchar *dirname;