Add q0 test statistic as valid option to CLI infer interface

The CLI API (e.g. `pyhf cls`) allows the test statistic to be set,
but it limits the options to `q` and `qtilde`. However, the discovery
test statistic `q0` is also a valid option here, but it can't currently
be set on the command line. This commit is a simple fix; it adds `q0`
to the list of allowed arguments to `pyhf cls --test-stat`.

Author: TC01

docs/contributors.rst

@@ -36,3 +36,4 @@ Contributors include:
 - Lorenz Gaertner
 - Melissa Weber Mendonça
 - Matthias Bussonnier
+- Ben Rosser

src/pyhf/cli/infer.py

@@ -144,7 +144,7 @@ def fit(
 @click.option('--measurement', default=None)
 @click.option('-p', '--patch', multiple=True)
 @click.option('--test-poi', default=1.0)
-@click.option('--test-stat', type=click.Choice(['q', 'qtilde']), default='qtilde')
+@click.option('--test-stat', type=click.Choice(['q', 'qtilde', 'q0']), default='qtilde')
 @click.option(
     '--calctype', type=click.Choice(['asymptotics', 'toybased']), default='asymptotics'
 )