refactor common test helpers into test/

This way we can share them with multiple tests

test/README.md

@@ -0,0 +1,36 @@
+# Testing
+
+NOTE: This directory contains test helpers. Actual tests are in the `src/`
+directory.
+
+
+## Running tests
+
+    yarn test
+
+Launches the test runner in the interactive watch mode.
+
+See the section about [running tests][tests] for more information.
+
+[tests]: https://facebook.github.io/create-react-app/docs/running-tests
+
+## Code coverage
+
+    yarn coverage
+
+Launches the test runner in the code coverage mode. Results are displayed in
+the terminal.
+
+    yarn coverage:html
+    xdg-open coverage/index.html
+
+Does the same thing except results are converted to more detailed html pages in
+the `coverage/` directory. (On macOS, use `open` and on Windows use `explorer`
+instead of `xdg-open` to open the html in your default web browser.)
+
+
+## Writing tests
+
+Tests are written using the [Jest][jest] testing framework.
+
+[jest]: https://jestjs.io/