Run tests on multiple CPU cores

We can make the tests run slightly faster by parallelizing them across
multiple CPU cores:

- from some casual testing locally, 2 cores gives the optimum speedup
- Travis container-based builds have 2 CPU cores available[1]
- the net gain is about 20%, or 2 seconds
- unfortunately we can’t do this on the API because each test is still
  using the same instance of the database

1. https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments

requirements_for_test.txt

@@ -3,6 +3,7 @@ pep8==1.7.0
 pytest==2.9.1
 pytest-mock==0.11.0
 pytest-cov==2.2.1
+pytest-xdist==1.14
 coveralls==1.1
 moto==0.4.23
 httpretty==0.8.14

scripts/run_tests.sh

@@ -33,7 +33,7 @@ display_result $? 2 "Front end code style check"
 export NOTIFY_ADMIN_ENVIRONMENT='config.Test'
 
 ## Code coverage
-py.test --cov=app --cov-report=term-missing tests/
+py.test -n2 --cov=app --cov-report=term-missing tests/
 display_result $? 3 "Code coverage"
 
 #py.test -v