Add more unit tests for client

[sibille]

Would be nice if we can increase the code coverage with some useful tests.

And more taking into account that most of them are commented out. See issue #1709

[jcoc611-microsoft]

@jsondoo is this one done?

[streetcoder123]

For this one, the testing environment and the build pipeline has been set. However, only 1 actual test has been written.

[jsondoo]

@sibille can this be closed?

[jsondoo]

Currently there are two unit tests, one for App.tsx and one for Card.tsx. We added a new command to package.json test: react-scripts test --coverage which uses the react-scripts Jest testing framework. This command is also run in our build pipeline