Make tox fail if a regression test output changes

.gitlab-ci.yml

@@ -6,6 +6,12 @@ cache:
   paths:
     - .cache/pip
 
+# Install git so that we can ensure that the regression test outputs remain
+# unchanged.
+before_script:
+  - apt-get update
+  - apt-get -qq -y install git
+
 # Run all of the test activities in tox.ini using Python 3.6.
 tox-36:
   image: python:3.6-slim

tox.ini

@@ -25,8 +25,12 @@ deps =
     pytest
     pytest-cov
     matplotlib>=1.5
+# NOTE: tox 3.18 deprecated whitelist_externals and added allowlist_externals.
+whitelist_externals =
+    git
 commands =
     {envpython} -bb {[base]pytest} {[base]test_paths} {posargs}
+    git diff --exit-code --stat tests/
 setenv =
     # Ignore ImportMismatchErrors when running doctests.
     # See https://github.com/pytest-dev/pytest/issues/2042 for details.