Fix output pipeline tests (unique Popen mocks)

tests/conftest.py

@@ -10,11 +10,15 @@ if TYPE_CHECKING:
 
 @pytest.fixture
 def Popen(mocker: 'pytest_mock.MockFixture'):
+    real_Popen = subprocess.Popen
+
+    def popen_factory(*args, **kwargs):
+        popen = mocker.create_autospec(real_Popen)
+        popen.stdin = open(os.devnull, "wb")
+        popen.stdout = open(os.devnull, "rb")
+        popen.wait.return_value = 0
+        return popen
+
     Popen = mocker.patch.object(subprocess, 'Popen', autospec=True)
-    popen = Popen.return_value
-
-    popen.stdin = open(os.devnull, "wb")
-    popen.stdout = open(os.devnull, "rb")
-    popen.wait.return_value = 0
-
+    Popen.side_effect = popen_factory
     yield Popen