test/dashboard: hardcode .coverage path to workaround tox bugs

test/dashboard: hardcode .coverage path to workaround tox bugs

rhel/centos 7.3 comes with tox 1.4.2, which has a bunch of issues related
to environment variable substitution. for instance, see
- https://github.com/tox-dev/tox/issues/515
so, instead of having different coverage files for py3 and py2.7, it
would be simpler to just use the same path for coverage. this approach
works because pytest-cov plugin erases the coverage data file at the
beginning of test[1].

---
[1] https://pypi.org/project/pytest-cov/#coverage-data-file .

Signed-off-by: Kefu Chai <kchai@redhat.com>