siso: keep hashfs ledger test paths in temp dir

The test created its fixture files under t.TempDir, but its non-Windows
state keys used paths rooted at /, such as /active_dir. HashFS treats an
absolute path as-is, so RemoveAll targeted /active_dir instead of the
fixture directory. This usually fails with a permission error, but it
could remove unrelated files if that directory exists and the test
process can write to it.

Build the persisted HashFS keys from the actual fixture paths so Remove
and RemoveAll stay inside the temporary directory on every platform. The
explicit paths also make the process-wide working directory change
unnecessary.

Change-Id: Ie84bf7f6444ae42a496facca973f49ba0efd47ba
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8130578
Auto-Submit: Philipp Wollermann <philwo@google.com>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>