Re-arrange `pathlib.Path` test methods in source code. No other changes.

The test methods are arranged in two groups. The first group checks
`stat()`, `open()`, `iterdir()`, `readlink()`, and derived methods like
`exists()`, `read_text()`, `glob()` and `resolve()`. The second group
checks all other `Path` methods. To minimise the diff I've maintained the
method order within groups where possible.

This patch prepares the ground for a new `_AbstractPath` class, which will
support methods in the first group above. By churning the test methods
here, subsequent patches will be easier to review and less likely to break
things.