`./x.py test tidy` should not count untracked paths towards entries limit

[JhonnyBillM]

When browsing folders in macOS' Finder app, the OS automatically creates a .DS_Store file in the folder you're browsing. .DS_Store is usually included in .gitignore by users, making it trickier to detect which file increased the entries count in a given folder when getting the following error:

tidy error: following path contains more than 948 entries, you should move the test to some relevant subdirectory (current: 949): …/rust/src/test/ui

Because of scenarios like this, and because of the original purpose of the entries check described in #73494, tidy should ignore untracked files and directories inside tests directories.

Related links:

• Check the number of entries in UI test on tidy #79959
• Zulip topic - Tidy error: path contains more than 948 entries
• x.py tidy doesn't ignore untracked files #75582

[chenyukang]

@rustbot claim