Convert test_custom grains test to pytest

Converts the test_custom grains test from an old style test to a pytest
test. This is just to make it easier for me to debug why it is failing
and needs to be done at some point anyway.

tests/filename_map.yml

@@ -155,7 +155,7 @@ salt/engines/*:
   - pytests.unit.engines.test_engines
 
 salt/grains/*:
-  - integration.grains.test_custom
+  - pytests.integration.grains.test_custom
 
 salt/matchers/*:
   - integration.states.test_match

tests/integration/grains/test_custom.py

@@ -1,23 +0,0 @@
-"""
-Test the core grains
-"""
-
-import pytest
-
-from tests.support.case import ModuleCase
-
-
-@pytest.mark.windows_whitelisted
-class TestGrainsCore(ModuleCase):
-    """
-    Test the core grains grains
-    """
-
-    @pytest.mark.slow_test
-    def test_grains_passed_to_custom_grain(self):
-        """
-        test if current grains are passed to grains module functions that have a grains argument
-        """
-        self.assertEqual(
-            self.run_function("grains.get", ["custom_grain_test"]), "itworked"
-        )

tests/pytests/integration/grains/test_custom.py

@@ -0,0 +1,20 @@
+"""
+Test the custom grains
+"""
+
+import pytest
+
+pytestmark = [
+    pytest.mark.windows_whitelisted,
+    pytest.mark.slow_test,
+]
+
+
+def test_grains_passed_to_custom_grain(salt_call_cli):
+    """
+    test if current grains are passed to grains module functions that have a grains argument
+    """
+    ret = salt_call_cli.run("grains.item", "custom_grain_test")
+    assert ret.returncode == 0
+    assert ret.data
+    assert ret.data["custom_grain_test"] == "itworked"