…lock

Given a `setup` block like:

    setup do
      {:ok, file: :foo}
    end

...ExUnit raises an exception, but it also overrides the test's
`:file` tag, which could cause problems for the manifest since it
looks up the existence of the file on the file system, and would
get an error if it was not a string.

This uses the ExUnit manifest to filter to only tests that failed
the last time they ran. As an optimization, we filter out files
that have no failures so we load the minimum set of files necessary.

Since that is the only information we care about, we do not
have to store every test with its status. This allows us to
vastly simplify the manifest, as we no longer have to use an
`:entry` record. Reading and writing the manifest should be
faster now, since the manifest tends toward being empty.

Since we are only storing failures, I have renamed the manifest
(and the `:manifest_file` option and file path) to indicate that
it deals only with failures.

- Rename manifest fail to indicate that mix owns the file
- Leave a TODO
- Document new ExUnit config options
- Fix ENV var typo