[test] Avoid alloca use as clang complain

Fixes https://circleci.com/gh/harfbuzz/harfbuzz/108171 complain

test/api/test-ot-color.c

@@ -210,7 +210,7 @@ static void
 test_hb_ot_color_palette_get_colors_v0 (void)
 {
   unsigned int num_colors = hb_ot_color_palette_get_colors (cpal_v0, 0, 0, NULL, NULL);
-  hb_color_t *colors = (hb_color_t*) alloca (num_colors * sizeof (hb_color_t));
+  hb_color_t *colors = (hb_color_t*) malloc (num_colors * sizeof (hb_color_t));
   size_t colors_size = num_colors * sizeof(*colors);
   g_assert_cmpint (num_colors, ==, 2);
 
@@ -252,6 +252,8 @@ test_hb_ot_color_palette_get_colors_v0 (void)
   g_assert_cmpint (num_colors, ==, 0);
   assert_color_rgba (colors, 0, 0x44, 0x44, 0x44, 0x44);  /* untouched */
   assert_color_rgba (colors, 1, 0x44, 0x44, 0x44, 0x44);  /* untouched */
+	
+  free (colors);
 }