pytester: use monkeypatch.chdir() for dir changing
The current method as the following problem, described by Sadra
Barikbin:
The tests that request both `pytester` and `monkeypatch` and use
`monkeypatch.chdir` without context, relying on `monkeypatch`'s teardown
to restore cwd. This doesn't work because the following sequence of
actions take place:
- `monkeypatch` is set up.
- `pytester` is set up. It saves the original cwd and changes it to a
new one dedicated to the test function.
- Test function calls `monkeypatch.chdir()` without context.
`monkeypatch` saves cwd, which is not the original one, before
changing it.
- `pytester` is torn down. It restores the cwd to the original one.
- `monkeypatch` is torn down. It restores cwd to what it has saved.
The solution here is to have pytester use `monkeypatch.chdir()` itself,
then everything is handled correctly.
