ui: Use sys.stdout and sys.stderr dynamically (#5729)

Before, we used to set it during initialization. But this
made it hard to test using capsys as capsys changes the
reference to sys.stdout and sys.stderr with the mocked one
during the test. But since we are using dvc.ui globally and
setting it to a real sys.stdout and sys.stderr before capsys
comes into effect, testing was not possible with it.

Now, by using property hack, we will be using the values of
sys.stdout and sys.stderr during the time of write, so the mocking
would work and would be easier to test.

tests/conftest.py

@@ -43,6 +43,13 @@ def reset_loglevel(request, caplog):
         yield
 
 
+@pytest.fixture(autouse=True)
+def enable_ui():
+    from dvc.ui import ui
+
+    ui.enable()
+
+
 @pytest.fixture(scope="session", autouse=True)
 def _close_pools():
     from dvc.fs.pool import close_pools