Fix build:test in rtc-store-dummy

[echarles]

in rtc-store-dummy, we have

yarn build:test
yarn run v1.22.4
$ tsc --build tests && cd tests && webpack

Insufficient number of arguments or no entry found.
Alternatively, run 'webpack(-cli) --help' for usage info.

Hash: 555cd1b116a775561239
Version: webpack 4.43.0
Time: 41ms
Built at: 18/07/2020 09:37:54

WARNING in configuration
The 'mode' option has not been set, webpack will fallback to 'production' for this value. Set 'mode' option to 'development' or 'production' to enable defaults for each environment.
You can also set it to 'none' to disable any default behavior. Learn more: https://webpack.js.org/configuration/mode/

ERROR in Entry module not found: Error: Can't resolve './build/index.spec.js' in '/Users/echar4/datalayer/repos/jupyterlab-rtc/packages/rtc-store-dummy/tests'
error Command failed with exit code 2.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.