Refactor tests to use builder

[casey]

This refactors the integration tests:

• Removes async code in favor of blocking code, since the tests don't need async, and blocking code is easier to write, and possibly compiles faster.
• Uses a builder pattern. AgoraTestContext::builder() creates a builder, which can then be customized with more method calls, and finally builde() constructs the test context. This should allow all customization to happen via the builder, which I find much easier to reason about vs the multiple nesting test functions we have currently
• Adds a bunch of helpers to AgoraTestContext, which allowed simplifying a bunch of tests quite a bit
• Moves some additional tests, although not many, into tests/integration.rs, which now use the builder pattern

[casey]

@soenkehahn This is a gargantuan PR, so I wanted to get it up. I'll probably keep working on it, and start moving the slow tests and browser tests.

[casey]

The bugfix_symlink_with_relative_base_directory is failing on windows. Here's the log. I gave up on it for now because this is a massive PR already, and porting tests one by one from here on out seems like a better idea.