eslint: Extend test-file import permissions to subdirectories.

Allow all files in subdirectories of `__tests__` to import dev-
dependencies, rather than just files in the `__tests__` directories
themselves.

.eslintrc.yaml

@@ -80,7 +80,7 @@ rules:
   import/prefer-default-export: off
   import/no-extraneous-dependencies:
   - error
-  - devDependencies: ['**/__tests__/*.js', tools/**]
+  - devDependencies: ['**/__tests__/**/*.js', tools/**]
   import/no-cycle: off  # This would be nice to fix; but isn't easy.
 
   # Jest