os: UserCacheDir and UserConfigDir have no tests

[bcmills]

os.UserCacheDir was added in https://go.dev/cl/78835 for #22536.

In examining #57630, it seems that this function is not covered by any tests at all. There were none added in https://go.dev/cl/78835, nor in https://go.dev/cl/120757.

It seems like we should at least have some test that verifies that UserCacheDir reports either an error or a directory in which the user is allowed to create files. 😅

(CC @rsc @robpike @ianlancetaylor @bradfitz @griesemer for os)

[gopherbot]

Change https://go.dev/cl/460917 mentions this issue: gopls/internal/lsp/filecache: use TempDir unless MkdirAll(UserCacheDir)

[gopherbot]

Change https://go.dev/cl/462041 mentions this issue: os: add test for UserCacheDir

[bcmills]

It appears that UserConfigDir (https://go.dev/cl/160877 / #29960) is similarly untested.

[gopherbot]

Change https://go.dev/cl/520262 mentions this issue: os: add tests for UserCacheDir and UserConfigDir