Default value for `timeout` is defined using a sta...

# tornadofx

[hastebrot]

Default value for timeout is defined using a static setTimeoutMillis() method in TestFX. This is important for test runs not to hang. A parameter for startupFunction is needed to make the function testable (for internal test suite), primaryStageProvider to make the primaryStage configurable, (re)using a single "primary" stage during test runs has some advantages. action: (Stage) -> Unit would actually provide the primary stage to setup the test stage.