Schema validation should use all installed resolvers...

Using the resolver of an actual AsdfFile context instead of the default
resolver allows any extensions that were installed as entry points to be
used as part of schema validation.

asdf/tests/test_asdf_schemas.py

@@ -165,8 +165,16 @@ def create_schema_example_id(argval):
             generate_example_schemas(),
             ids=create_schema_example_id)
 
+@pytest.fixture(scope='module')
+def asdf_resolver():
+    """Using the resolver of an actual AsdfFile context instead of the default
+    resolver allows any extensions that were installed as entry points to be
+    used during the schema validation process.
+    """
+    ctx = asdf.AsdfFile()
+    return ctx.resolver
 
-def test_validate_schema(schema_path):
+def test_validate_schema(schema_path, asdf_resolver):
     """Pytest to check validity of schema file at given path
 
     Parameters:
@@ -177,7 +185,7 @@ def test_validate_schema(schema_path):
     'parametrize' utility in order to account for all schema files.
     """
     # Make sure that each schema itself is valid.
-    schema_tree = schema.load_schema(schema_path, resolve_references=True)
+    schema_tree = schema.load_schema(schema_path, resolver=asdf_resolver, resolve_references=True)
     schema.check_schema(schema_tree)