Wait for dev bundle build in waitForVaadin

[mshabarov]

Vaadin Flow 24.3 has now a "Building front-end development bundle" spinner shown on the page when dev bundle is being generated, see this issue.

TestBench should take this into account in waitForVaadin method to let Flow apps to properly wait until custom dev bundle is ready.

Possible solution is to wait until spinner div

<div class="message">Building front-end development bundle</div>

disappears, like shown below:

waitUntil(driver -> {
            try {
                TestBenchElement testBenchElement = $("div").attribute("class", "message").waitForFirst();
                return testBenchElement == null;
            } catch (TimeoutException e){
                return true;
            } catch (NoSuchElementException e){
                return true;
            }
        });

[TatuLund]

I have used slightly different wayt to do the same

https://github.com/TatuLund/bean-table/blob/v24/src/test/java/org/vaadin/tatu/AbstractViewTest.java#L123

[mshabarov]

UPD: TestBench waitForVaadin method already has a check for dev server here.

This checks for a flag devServerIsNotLoaded which is added on "dev mode not ready" page here.

Since the custom dev bundle build shows the same dev-mode-not-ready page now, I expect that the existing TestBench implementation works properly.

We should start with testing the waitForVaadin() method properly waiting until custom dev bundle build in Spring+Flow skeleton starter project with JS customisations.