Check if this fixes the plugin already loaded issue

Author: jammer87

tests/cases/issues/test_issue_374.py

@@ -32,7 +32,7 @@ def test_something(self, app): pass
 
 def test_overridden_fixtures(pytester):
     pytester.makepyfile(OVERRIDDEN_FIXTURES_TEST_FILE)
-    result = pytester.runpytest("-p", "pytest_cases.plugin")
+    result = pytester.runpytest()
     result.assert_outcomes(passed=2)
 
 
@@ -87,5 +87,5 @@ def test_something(self, msg): pass
 
 def test_overridden_unions(pytester):
     pytester.makepyfile(OVERRIDDEN_UNION_FIXTURES_TEST_FILE)
-    result = pytester.runpytest("-p", "pytest_cases.plugin")
+    result = pytester.runpytest()
     result.assert_outcomes(passed=6)