releasetools: Update tests to match SparseImage change.

The previous change in commit 22632cc8
changed the behavior in loading file map. It now always puts a copy of
the input text in `extra` field. Update the tests accordingly.

Bug: 79951650
Test: Use a lunch'd target; `atest --host releasetools_test`
Change-Id: Iccf06c817c1305bf9946d7759c6f6f6af21fe85e