Consider making `fscacher.test` self-contained

I was trying to solve the issue discussed in https://bugs.debian.org/1079398#10 . For Debian's autopkgtest infra to work properly, the following patch will be needed, as discussed in https://lists.debian.org/debian-python/2024/11/msg00016.html :

```diff
--- fscacher-0.4.1.orig/src/fscacher/tests/test_cache.py
+++ fscacher-0.4.1/src/fscacher/tests/test_cache.py
@@ -8,8 +8,8 @@ import subprocess
import sys
import time
import pytest
-from .. import PersistentCache
-from ..cache import DirFingerprint, FileFingerprint
+from fscacher import PersistentCache
+from fscacher.cache import DirFingerprint, FileFingerprint

platform_system = platform.system().lower()
on_windows = platform_system == "windows"
--- fscacher-0.4.1.orig/src/fscacher/tests/test_util.py
+++ fscacher-0.4.1/src/fscacher/tests/test_util.py
@@ -1,5 +1,5 @@
import pytest
-from ..cache import xor_bytes
+from fscacher.cache import xor_bytes

@pytest.mark.parametrize(
```

I believe the logic is that tests should be self-contained, and not a subpackage under the `fscacher` namespace that utilizes relative import. Now I am wondering if that patch makes sense to you.

Besides, with this patch, the `/src/fscacher/tests/` directory can also be moved to the top dir and not nested under `/src/fscacher/`. That is just a wishlist item and you can decide whether that is useful.