tests: simplify doctest execution

The following cookiecutter change:

https://github.com/inveniosoftware/cookiecutter-invenio-module/pull/98

should be propagated to this Invenio module.

Namely, in `run-tests.sh`, the sphinx for doctests is invoked after pytest run:

```console
$ tail -3 ./\{\{\ cookiecutter.project_shortname\ \}\}/run-tests.sh
sphinx-build -qnNW docs docs/_build/html && python setup.py test && sphinx-build -qnNW -b doctest docs docs/_build/doctest
```

This sometimes led to problems on Travis CI with the second sphinx-build run due
to "disappearing" dependencies after the example application was tested.

A solution that worked for invenio-marc21 (see
https://github.com/inveniosoftware/invenio-marc21/pull/49#issuecomment-281287103)
and that was integrated in cookiecutter (see
https://github.com/inveniosoftware/cookiecutter-invenio-module/pull/98) was to
run doctest execution in pytest, removing the second `sphinx-build` invocation.

This both solved Travis CI build failures and simplified test suite execution.

Note that this change may necessitate to amend the code tests etc so that things
would be executed with the Flask application context (see
https://github.com/inveniosoftware/invenio-marc21/pull/49/commits/09e98fc18fd5591dc65d806286e8da51b7e384b4).

Start with run-tests.sh and compare its current test sequence with the cookiecutter change linked in the issue. Move doctest execution into pytest, remove the second sphinx-build invocation, and run the test suite to identify tests needing Flask application context. Done means the suite executes doctests through pytest without the redundant Sphinx run.

Written by the indexing model from the issue text.