Add sslib master tox build + flesh out test docs

Add a tox build that runs tests against securesystemslib's tip of
development, i.e. master branch, to ease preparation of tuf for a
new securesystmeslib release.

The tox build is run on travis but is allowed to fail.

This commit also fleshes out the testing section of the
contribution documentation.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

.travis.yml

@@ -12,6 +12,12 @@ matrix:
       env: TOXENV=py35
     - python: "3.6"
       env: TOXENV=py36
+    - python: "3.6"
+      env: TOXENV=with-sslib-master
+
+  allow_failures:
+    - python: "3.6"
+      env: TOXENV=with-sslib-master
 
 install:
   - pip install tox coveralls

docs/CONTRIBUTORS.rst

@@ -99,6 +99,7 @@ To work on the TUF project, it's best to perform a development install.
    optional cryptographic support, the testing/linting dependencies, etc.
    With a development installation, modifications to the code in the current
    directory will affect the installed version of TUF.
+
 ::
 
     $ pip install -r dev-requirements.txt
@@ -107,9 +108,46 @@ To work on the TUF project, it's best to perform a development install.
 Testing
 =======
 
-The Update Framework's unit tests can be executed by invoking
-`tox <https://testrun.org/tox/>`_. All supported Python versions are
-tested, but must already be installed locally.
+The Update Framework's unit test suite can be executed by invoking the test
+aggregation script inside the *tests* subdirectory. ``tuf`` and its
+dependencies must already be installed (see above).
+::
+
+    $ cd tests
+    $ python aggregate_tests.py
+
+
+To run the tests, measuring code coverage, the script can be run with the
+``coverage`` tool.
+::
+
+    $ coverage run aggregate_tests.py && coverage report
+
+
+To develop and test ``tuf`` with above commands alongside its in-house dependency
+`securesystemslib <https://github.com/secure-systems-lab/securesystemslib>`_,
+it is recommended to first make an editable install of ``securesystemslib`` (in
+a *venv*), and then install ``tuf`` in editable mode too (in the same *venv*).
+::
+
+    $ cd path/to/securesystemslib
+    $ pip install -r dev-requirements.txt
+    $ cd path/to/tuf
+    $ pip install -r dev-requirements.txt
+
+
+With `tox <https://testrun.org/tox/>`_ the test suite can be executed in a
+separate *venv* for each supported Python version. While the supported
+Python versions must already be available, ``tox`` will install ``tuf`` and its
+dependencies anew in each environment.
 ::
 
     $ tox
+
+
+An additional non-default ``tox`` environment is available and can be used to
+test ``tuf`` against the tip of development of ``securesystemslib`` on GitHub,
+to e.g. prepare the former for a new release of the latter.
+::
+
+    $ tox -e with-sslib-master

tox.ini

@@ -26,3 +26,16 @@ deps =
     --editable {toxinidir}
 
 install_command = pip install --pre {opts} {packages}
+
+
+# Develop test env to run tests against securesystemslib's master branch
+# Must to be invoked explicitly with, e.g. `tox -e with-sslib-master`
+[testenv:with-sslib-master]
+deps =
+    --editable git+http://github.com/secure-systems-lab/securesystemslib.git@master#egg=securesystemslib[crypto,pynacl]
+    -r{toxinidir}/ci-requirements.txt
+    --editable {toxinidir}
+
+commands =
+    coverage run aggregate_tests.py
+    coverage report -m