Co-authored-by: deReeperJosh <joshua@dereeper.co.nz>

Don't use reference variable bound to a temporary value

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Check return value of calls to File::CreateTempDir() from CoreTiming,
FileSystem, and MMIO test classes to verify the test user directory
exists, and fail the tests otherwise.

This fixes CreateFullPath to not create directories when it is known
that they already exist, instead of calling CreateDirectory anyway
and checking if the error is AlreadyExists. (That doesn't work
now that we have an accurate implementation of CreateDirectory
that performs permission checks before checking for existence.)

I'm not sure what I was thinking when I wrote that function.

Also adds some tests for CreateFullPath.

* Test recursive directory deletion
* Test "in use" check for both files and directories

Files cannot be given a different file name, only moved across
directories.

Add a test for that behaviour and fix the existing
RenameWithExistingTargetFile test.