fix: Use function scope to avoid altering hypotest_args fixture (#2011)

* Use the 'function' scope for hypotest_args fixture. Fixing the POI in the model config
  changes it for the entire fixture if the fixture is 'module' scope, which would cause
  errors in other tests that later use it and defeats the point of the fixture.
* Remove unnecessary use of tmpdir fixture as no output is written.

tests/test_infer.py

@@ -4,7 +4,7 @@
 import scipy.stats
 
 
-@pytest.fixture(scope='module')
+@pytest.fixture(scope="function")
 def hypotest_args():
     pdf = pyhf.simplemodels.uncorrelated_background(
         signal=[12.0, 11.0], bkg=[50.0, 52.0], bkg_uncertainty=[3.0, 7.0]
@@ -466,16 +466,16 @@ def test_toy_calculator(tmpdir, hypotest_args):
     )
 
 
-def test_fixed_poi(tmpdir, hypotest_args):
+def test_fixed_poi(hypotest_args):
     """
     Check that the return structure of pyhf.infer.hypotest with the
     addition of the return_expected keyword arg is as expected
     """
 
-    _, _, pdf = hypotest_args
-    pdf.config.param_set('mu').suggested_fixed = [True]
+    test_poi, data, model = hypotest_args
+    model.config.param_set("mu").suggested_fixed = [True]
     with pytest.raises(pyhf.exceptions.InvalidModel):
-        pyhf.infer.hypotest(*hypotest_args)
+        pyhf.infer.hypotest(test_poi, data, model)
 
 
 def test_teststat_nan_guard():