Start using intellij-ui-test-robot for testing

[SingingBush]

Jetbrains have released a testing framework similar to Selenium WebDriver for testing Intellij based plugins: https://github.com/JetBrains/intellij-ui-test-robot

It would be good to start using it to test the project as there are many areas that cannot be tested with typical Junit tests. Potentially this may need to be another sub-project that is run independently against the plugin when it's being run within Intellij.

    var uiTestVersion = '0.11.19'
    testImplementation "com.intellij.remoterobot:remote-robot:${uiTestVersion}"
    testImplementation "com.intellij.remoterobot:remote-fixtures:${uiTestVersion}"