tests: Use options and fixtures to control all browsers

[jsfehler]

This PR refactors the tests to:

• Remove the class based-structure and replace it with browser flags and isolated fixtures.
• Replace an autouse fixture for visiting the dummy site with an explicit call in each test. This makes it easier to understand each test in isolation.
• Run more tests for each browser.
• Use a standard structure for xfail and skip instead of the macos tests subclassing tests.
• Use a fixture to get the dummy app's URL instead of importing from it's internals.
• Improve test organization by splitting them into more files and directories based on what they do.
• Remove unnecessary imports from the requirements file for the tests

Before vs This PR:

• Firefox: 446 vs 504
• Chrome: 443 vs 498
• Edge: 443 vs 498
• Remote Chrome & Firefox: 380 + 2 skip vs 496
• Remote Safari: 139 + 1 skip + 14 xfail + 41 error vs 229 + 3 skip + 13 xfail + 3 xpass
• Django + flask + zope: 498 + 7 skip vs 499

[fsouza]

woah this is large, but generally looks good. I'm approving after skimming through the code and given that CI is green 😁

[jsfehler]

woah this is large, but generally looks good. I'm approving after skimming through the code and given that CI is green 😁

Yeah, sorry about that. I did as much as I could over separate PRs but getting it done in one shot made addressing issues a lot easier without putting in temporary hacks.