[meta] Add backwards compatibility layer to browsertestbase to ease conversion from webtestcase

Well, we already have this BC layer, we just expand it over time.

Ignore my previous comment, well this is just a tracking issue.

Another one: #2795607: Provide a settingsSet method on BrowserTestBase but use an @deprecated tag on it

Another novice one: #2795611: Provide a assertHeader legacy assertion

I have been converting some tests to BTB and I noticed a few small but common changes that had to be made several times. I'm wondering if they should be fixed in BTB rather than the individual tests, if only temporarily for this issue and #2770921: Feb 21st: Convert chunk of WTB to BTB by just moving classes, changing use statements adding traits.

• Most drupalPostAjaxForm() calls can be changed to drupalPostForm() without any side effects. BTB could implement it as an alias.
• Calls to dropalPostForm() that use NULL as the second parameter need to be changed to [] or array(). BTB could accept NULL as equivalent.
• I have been changing calls to clickLink() that use the second argument ($index) to click() and specifying the link via CSS selectors because BTB ignores the second argument. Support for it could be re-added to BTB.

Thoughts? I could create separate issues if these seem like good ideas.

Good ideas, I'm not sure about drupalPostAjaxForm() because those tests should really be converted to JavascriptTestBase if they need to test AJAX functionality. Yes, please create those child issues!

@jmuzz++

Opened #2814047: BrowserTestBase should implement AssertMailTrait for backwards compatibility

Oh, and we have decided to NOT provide compatibility for drupalPostAjaxForm(). Instead, we will convert those tests doing AJAX to proper JavasscriptTestBase tests. See #2809161: Convert Javascript/AJAX testing to use JavascriptTestBase.

Added #2820182: Add a LegacyBrowserTestBase which uses every legacy trait we have to make things a bit easier with the trait conversions.

Opened #2862470: Add assertOptionByText() to AssertLegacyTrait for better browser test compatibility.

I think we have a path for individual conversions now, so I don't consider this as be longer useful. Feel free to disagree and reopen the issue :)

There are still some pending child issues...