cacheprovider: use warnings directly

Allows for filtering of PytestCacheWarning.

Using `_issue_warning_captured` is not necessary here, and was probably
only used because the cacheprovider misses warnings during
`pytest_sessionfinish`, which is also fixed here.

I think the usage of `_issue_warning_captured` can be removed/reduced
further, but also that this is good enough for now.

Ref: https://github.com/pytest-dev/pytest/issues/6681.

testing/test_cacheprovider.py

@@ -56,9 +56,7 @@ class TestNewAPI:
             testdir.tmpdir.ensure_dir(".pytest_cache").chmod(mode)
 
     @pytest.mark.skipif(sys.platform.startswith("win"), reason="no chmod on windows")
-    @pytest.mark.filterwarnings(
-        "ignore:could not create cache path:pytest.PytestWarning"
-    )
+    @pytest.mark.filterwarnings("default")
     def test_cache_failure_warns(self, testdir, monkeypatch):
         monkeypatch.setenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD", "1")
         cache_dir = str(testdir.tmpdir.ensure_dir(".pytest_cache"))