typeshed: run stubtest in CI (#3727)

* README.md: refactor "Running the tests"

This organises the section a little better. Previously some tests were
unmentioned; it read as if mypy_test and pytype_test were the only
tests. The section is now organised by test, making it easy to keep
track of the requirements and details of each. This also makes it
easier to add documentation for stubtest.

Also mention turning on Travis CI on your fork, since that is very
useful.

* README.md: document stubtest_test.py

* stubtest_test: add it

* travis: add stubtest_test to CI

* stubtest_test: add whitelists

.travis.yml

@@ -29,3 +29,19 @@ jobs:
     - name: "flake8"
       install: pip install -r requirements-tests-py3.txt
       script: flake8
+    - name: "stubtest py38"
+      python: 3.8
+      install: pip install -U git+git://github.com/python/mypy@a07dbd00
+      script: ./tests/stubtest_test.py
+    - name: "stubtest py37"
+      python: 3.7
+      install: pip install -U git+git://github.com/python/mypy@a07dbd00
+      script: ./tests/stubtest_test.py
+    - name: "stubtest py36"
+      python: 3.6
+      install: pip install -U git+git://github.com/python/mypy@a07dbd00
+      script: ./tests/stubtest_test.py
+    - name: "stubtest py35"
+      python: 3.5
+      install: pip install -U git+git://github.com/python/mypy@a07dbd00
+      script: ./tests/stubtest_test.py