Add sphinx-testing to g-r

sphinx-testing is needed by os-api-ref for testing the theme.

- Is the library actively maintained?
  No changes in the last 12 months, but also no open issues.

- Is the library good code?
  I believe so, includes tests itself.

- Is the library python 3 compatible?
  Yes.

- Is the library license compatible?
  Yes, BSD license.

- Is the library already packaged in the distros we target (Ubuntu
  latest / Fedora latest)?
  Packages found for Ubuntu and openSUSE:
  http://packages.ubuntu.com/xenial/python-sphinx-testing
  https://build.opensuse.org/package/show/devel:languages:python/python-sphinx-testing

- Is the function of this library already covered by other libraries
  in ``global-requirements.txt``?

  Not that I'm aware of.

- Is the library required for OpenStack project or related dev or
  infrastructure setup? (Answer to this should be Yes, of course)
  Which?

  Yes, os-api-ref - which is used to display api-ref of server projects.

Change-Id: I3bb1d25e3b9881b9cf18134f0eac84a8f4aedae6

global-requirements.txt

@@ -357,6 +357,7 @@ selenium>=2.50.1  # Apache-2.0
 # situations as progressive releases of projects are done.
 setuptools>=16.0,!=24.0.0  # PSF/ZPL
 sphinx>=1.2.1,!=1.3b1,<1.4  # BSD
+sphinx-testing # BSD License
 sphinxcontrib-blockdiag  # BSD
 sphinxcontrib-httpdomain  # BSD
 sphinxcontrib-seqdiag  # BSD

upper-constraints.txt

@@ -354,6 +354,7 @@ singledispatch===3.4.0.3;python_version=='2.7'
 six===1.10.0
 smmap===0.9.0
 spec-cleaner===0.9.0
+sphinx-testing===0.7.1
 sphinxcontrib-blockdiag===1.5.5
 sphinxcontrib-httpdomain===1.5.0
 sphinxcontrib-pecanwsme===0.8.0