build: Require test file names to include (spec|test)

The default Jest config was considering my test fixture code as a
test suite because it was located under the `__tests__` directory.

Now, test files must end with `.test.ts` (or any of its common
variants. As a result, fixture code, which ends with `.ts` is no
longer considered a test suite.

jest.config.js

@@ -1,3 +1,4 @@
 module.exports = {
   preset: 'react-native',
-};
+  testMatch: ['**/?(*.)+(spec|test).[jt]s?(x)'],
+};