Ensure platform is not maintained between tests

This removes the `_instance` on the platform object that we use
for caching and not recreating the object everytime at the start
of every test.

This is to ensure our tests share the least amount of state.

The performance penalty is from 2 to 5% accross the whole test suite.
The readings were done 5 times for each before and after the change
and on the same computer.

conftest.py

@@ -23,6 +23,8 @@ import shutil
 
 import pytest
 
+from buildstream._platform.platform import Platform
+
 
 def pytest_addoption(parser):
     parser.addoption('--integration', action='store_true', default=False,
@@ -52,3 +54,8 @@ def integration_cache(request):
         shutil.rmtree(os.path.join(cache_dir, 'artifacts'))
     except FileNotFoundError:
         pass
+
+
+@pytest.fixture(autouse=True)
+def clean_platform_cache():
+    Platform._instance = None