Add a LegacyBrowserTestBase which uses every legacy trait we have

+++ b/core/tests/Drupal/Tests/LegacyBrowserTestBase.php
@@ -0,0 +1,45 @@
+use Behat\Mink\Driver\GoutteDriver;
+use Behat\Mink\Element\Element;
+use Behat\Mink\Mink;
...

Are all the unused use statements needed?

I would avoid adding yet another test base class.

Why is it such a problem that we add the most common APIs to BrowserTestBase itself? Code separation we can do with traits, so BrowserTestBase growing in code size is not an argument. For discoverability I think BrowserTestBase should have a rich API with many helper methods.

If we convert the old Simpletests to LegacyBrowserTestBase then we need to convert them another time to BrowserTestBase after that. I think it just creates more work and confusion.

But anyway, I also don't care too much about this, so feel free to proceed if others agree.

This is much better than polluting BTB itself!

Not sure that we need this now. In #2814047: BrowserTestBase should implement AssertMailTrait for backwards compatibility we decided that we can do the conversion script from old Simpletests to BrowserTestBase in a way that we add the required trait if the method gets called. That was successful for AssertMailTrait::getMails() in #2770921: Feb 21st: Convert chunk of WTB to BTB by just moving classes, changing use statements adding traits.

If we agree that approach is good enough then we can close this as won't fix.

Having that class in core means we have to maintain it, which means we have to test it although we don't use it in core. I don't see the benefit of doing that work. Probably some input from contrib maintainers would be good at this point, would they use LegacyBrowserTestBase or would they convert straight to BrowserTestBase directly?

@tim.plunkett how would you like to convert your tests?

Within a single subsystem or module, I'd prefer to individually convert each test to the latest and greatest approach. An intermediate step would just mean extra work.

But in terms of the best or most efficient way to remove the old WebTestBase, a legacy base class makes sense.

Absolutely we should strive to keep BTB clean.

Just to clarify: we will not remove WebTestBase in the Drupal 8 release cycle, ever. So Drupal 8 contrib converting their tests is optional. Simpletest will be supported forever in Drupal 8.

In Drupal 9 we will remove WebTestBase, but of course we will also remove any LegacyBrowserTestBase. So if a d8 contrib module converted to LegacyBrowserTestBase it will have to convert again to BrowserTestBase in Drupal 9.

Having that class in core means we have to maintain it, which means we have to test it although we don't use it in core. I don't see the benefit of doing that work.

I agree with this.

Adding a trait to a specific test class when a method is needed is pretty trivial, and finding the right trait isn't all that hard either.

So my 2cts, I'd not add this.