Allow either slash in path passed to Jest

Jest takes an argument to specify which tests should be run. The
argument is treated as a regular expression and matched against the
filenames of potential tests. Unfortunately Jest does not normalize path
separators to forward slashes when running this comparison, so (for
example) `tests/unit` doesn't match anything on Windows.

This change replaces the forward slash in Jest arguments with `[\\\\/]`.

Ideally, that should just be `[\\/]` -- escaping the backslash so that
it survives NPM's JSON parsing -- but unfortunately the Windows version
of Jest parses its command line arguments differently from other
platforms. Specifically, the Windows version of Jest processes
backslashes one extra time. Fortunately having a redundant backslash is
OK on Mac and Linux, so `[\\\\/]` ends up working on all platforms.

package.json

@@ -11,8 +11,8 @@
     "i18n:src": "babel src > tmp.js && rimraf tmp.js && ./scripts/build-i18n-source.js ./translations/messages/ ./translations/",
     "lint": "eslint . --ext .js,.jsx",
     "start": "npm run i18n:msgs && webpack-dev-server",
-    "unit-test": "jest test/unit",
-    "integration-test": "npm run build && jest --runInBand test/integration",
+    "unit-test": "jest test[\\\\/]unit",
+    "integration-test": "jest --runInBand test[\\\\/]integration",
     "test": "npm run lint && npm run unit-test && npm run integration-test",
     "watch": "webpack --progress --colors --watch"
   },