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Test installed version of package

To test against the installed version of the package (which is
preferably since this can catch installation file inclusion bugs) we
have to deal with quirks of coverage.py. By default the ecosystem is
set up to do development coverage tests (e.g. in src, or by using `pip
install -e .` or `python setup.py develop`).

We want to test against the version installed, to do this we have to
find the install path which we'll then pass to the `--cov` arg added by
`pytest-cov`. To do this, we have to cd out of the current folder, and
import the installed version and get it's install path (if we don't cd
out, then we end up getting the existing directory in the cwd since by
default cwd is on `sys.path`)

Once we have the install path, we pass that to the `--cov` of pytest,
however we also want to rewrite the paths for codecov to pick them up on
the website, if they don't have to code coverage with local pat...