Backfill tests around whether dependencies are re-saved

We try to prevent the HTML/markdown reports from being updated with a
new timestamp every time you run license_finder.  The timestamp is based
on when the DB was last changed (a questionable approach... see bug
59043220) so we try to avoid changing the DB unnecessarily. These tests
assert that when a dependency's license isn't changing, the DB won't be
updated.
Move db transaction to where it will make a difference

Wrapping a single save in a transaction isn't useful.  Better to wrap
groups of saves and/or deletes.
Rename local vars
Merge pull request #97 from mainej/fix-transactions

Fix transaction handling