We previously had some tests for some happy paths and a few specific
failures into an empty directory with no existing locks, but we didn't
have tests for the installer respecting existing lock file entries.
This is a start on a more exhaustive set of tests for the installer,
aiming to visit as many of the possible codepaths as we can reasonably
test using this mocking strategy. (Some other codepaths require different
underlying source implementations, etc, so we'll have to visit those in
other tests separately.)
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