Use the default coverage report file `.coverage` for unit and functional
tests, running Augur in functional tests through an alias that runs
coverage in append mode. Then, use the coverage xml command to generate
the XML report needed by the codecov tool.

…I workflow

This avoids baking in coverage to the tests and lets it remain a
separate concern (e.g. CI's concern).  In my experience it's better to
have coverage tracing happen outside of tests (i.e. the tests are
unaware) rather than as integrated into the tests themselves.

Partially reverts commit 723d609, which
added code for coverage to each cram test file.

I chose to use the github.workspace context var in the CI workflow so I
could use the "env:" block for the job step.  Alternatively, we could
use `realpath` to construct an `env AUGUR=… …` call chaining to the
`cram …` invocation, but that seemed slightly messier to me.