As suggested here: https://dolp.in/pr7059#pullrequestreview-125401778
More descriptive than having a std::tuple of FS::Mode, and lets us
give names to known triplets of modes (like in ES). Functions that
only forward mode arguments are slightly less verbose now too.
The existing backend did not handle cases where the target exists
correctly.
This is a bug that has been around forever but was only recently
exposed when ES started to use our FS code.
Also adds some unit tests to make sure this won't get broken again.
This adds unit tests for the Wii filesystem now that the filesystem
interface is neatly separated from the IPC code.
Basic FS functionality is tested, in addition to problematic usages and
edge cases that Dolphin used to handle incorrectly (which of course
broke emulated software).
These tests should make it quite a bit harder to introduce regressions.
Issues that are covered by the tests in particular:
* Metadata: issue 10234 (though tests are commented out for now because
Dolphin doesn't support NAND images yet so it can't track metadata);
* EOF seeks/reads: https://github.com/dolphin-emu/dolphin/pull/4942
* Read/write operations from multiple handles: see issue 2917, 5232 and
8702 and https://github.com/dolphin-emu/dolphin/pull/2649
Léo Lam