Fix unit tests on macOS

* Libvirt tests: Mocked the platform check to prevent failures
  during class initialization on macOS.
* test_get_available_resource: Mocked filesystem access
  to handle the absence of /proc/meminfo on macOS.

Change-Id: I04e1197d9f77c1acd6a7f63120446615c3fd133e

nova/tests/fixtures/libvirt.py

@@ -2477,6 +2477,8 @@ class LibvirtFixture(fixtures.Fixture):
             'Linux', '', '5.4.0-0-generic', '', obj_fields.Architecture.X86_64)
         self.mock_uname = self.useFixture(
             fixtures.MockPatch('os.uname', return_value=fake_uname)).mock
+        self.useFixture(fixtures.MonkeyPatch(
+            'nova.virt.libvirt.driver.sys.platform', 'linux'))
 
         real_exists = os.path.exists
 

nova/tests/unit/virt/libvirt/test_driver.py

@@ -22388,6 +22388,9 @@ class TestGuestConfigSysinfoSerialOS(test.NoDBTestCase):
         # Don't initialise the Host
         self.useFixture(fixtures.MockPatch('nova.virt.libvirt.driver.host'))
 
+        self.useFixture(fixtures.MonkeyPatch(
+            'nova.virt.libvirt.driver.sys.platform', 'linux'))
+
     def _test_get_guest_config_sysinfo_serial(self, expected_serial):
         drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
 

nova/tests/unit/virt/test_virt_drivers.py

@@ -607,8 +607,10 @@ class _VirtDriverTestCase(_FakeDriverBackendTestCase):
 
     @catch_notimplementederror
     def test_get_available_resource(self):
-        available_resource = self.connection.get_available_resource(
-                'myhostname')
+        with mock.patch('nova.virt.libvirt.host.Host._get_avail_memory_kb',
+                        mock.Mock(return_value=1024)):
+            available_resource = self.connection.get_available_resource(
+                    'myhostname')
         self._check_available_resource_fields(available_resource)
 
     @catch_notimplementederror