test: update prefix for model lookups (#2286)

Followup to #2265, accommodating MODFLOW-ORG/modflow-devtools#207

Author: wpbonelli

autotest/test_largetestmodels.py

@@ -10,7 +10,7 @@
 )
 from framework import TestFramework
 
-MODELS = [m for m in models.get_models().keys() if m.startswith("large/")]
+MODELS = [m for m in models.get_models().keys() if m.startswith("mf6/large/")]
 SKIP = [
     "test1002_biscqtg_disv_gnc_nr_dev",
     "test1002_biscqtg_disv_nr_MD_dev",

autotest/test_testmodels_mf6.py

@@ -10,7 +10,7 @@
 )
 from framework import TestFramework
 
-MODELS = [m for m in models.get_models().keys() if m.startswith("test/")]
+MODELS = [m for m in models.get_models().keys() if m.startswith("mf6/test/")]
 SKIP = [
     "alt_model",
     "test205_gwtbuy-henrytidal",