[Fix] Make checkDiskSpace look at the correct drive again

[CommandMC]

In my checkDiskSpace refactor (#3440), I used the root property of the object returned by Node's path.parse, thinking that it'd, logically, give me the root folder (or in other words the mount point) of the disk the path is on. This is not the case on Linux/macOS, it's just simply always / there.
This issue would manifest itself in the wrong disk space info being displayed when installing a game onto a drive other than the primary one.

Resolving this was somewhat tricky. Since this function runs on both macOS and Linux, we can't use commandline tools like findmnt to help us here, so I went back to what we did before the refactor: Going up the directory tree and using the first path that exists. This works just fine on Unix, since there will always be a / path (as opposed to Windows, where entire drive letters can be missing)

I've also added a test for these functions now, so this issue can't happen again.

To test this:

1. Install a game onto another hard drive on the current main branch, see that the total/free disk space is reported incorrectly
2. Try the same thing with this PR, info will now be correct

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[Etaash-mathamsetty]

looks like it's working again