[sonic-py-common] Clear environment variables before running device_info tests (#7273)

#### Why I did it

To ensure any environment variables which are configured in the build/test environment do not influence the behavior of sonic-py-common during unit tests. For example, variables which might be set by continuous integration pipelines.

#### How I did it

Add class-scoped pytest fixture to `TestDeviceInfo` class which stashes the current environment variables, clears them and yields. Once all the test cases in the class finish, the fixture will restore the original environment variables.

Also remove unnecessary unittest-style setup and teardown functions from interface_test.py

src/sonic-py-common/tests/device_info_test.py

@@ -9,6 +9,8 @@ else:
     # https://pypi.python.org/pypi/mock
     import mock
 
+import pytest
+
 from sonic_py_common import device_info
 
 from .mock_swsssdk import SonicV2Connector
@@ -51,9 +53,12 @@ EXPECTED_GET_MACHINE_INFO_RESULT = {
 }
 
 class TestDeviceInfo(object):
-    @classmethod
-    def setup_class(cls):
-        print("SETUP")
+    @pytest.fixture(scope="class", autouse=True)
+    def sanitize_environment(self):
+        # Clear environment variables, in case a variable is set in the test
+        # environment (e.g., PLATFORM) which could modify the behavior of sonic-py-common
+        with mock.patch.dict(os.environ, {}, clear=True):
+            yield
 
     def test_get_machine_info(self):
         with mock.patch("os.path.isfile") as mock_isfile:

src/sonic-py-common/tests/interface_test.py

@@ -4,10 +4,6 @@ import sys
 from sonic_py_common import interface
 
 class TestInterface(object):
-    @classmethod
-    def setup_class(cls):
-        print("SETUP")
-
     def test_get_interface_table_name(self):
         result = interface.get_interface_table_name("Ethernet0")
         assert result == "INTERFACE"
@@ -35,7 +31,3 @@ class TestInterface(object):
         assert result == "VLAN_INTERFACE"
         result = interface.get_port_table_name("Loopback0")
         assert result == "LOOPBACK_INTERFACE"
-
-    @classmethod
-    def teardown_class(cls):
-        print("TEARDOWN")