Tests crash if os.UserCacheDir() returns an unusable directory

[skitt]

Since commit 58c17f6 (“addr.Suggest should lock a file instead of memory”), pkg/internal/testing/addr/manager.go’s init() function tries to create a directory using either os.UserCacheDir() or os.TempDir(), whichever succeeds first. In many build environments, $HOME is non-empty but points to an unusable directory; in such cases, os.UserCacheDir() returns an unusable directory, which causes init() to panic.

It could instead try os.TempDir(), which is usable in many more scenarios than os.UserCacheDir().