docs: Need more guidance on when to use substitutions vs get_host_info()

[rakuco]

#36088 got me curious about the topic, and I couldn't find much in the documentation.

Namely, https://web-platform-tests.org/writing-tests/server-pipes.html and https://web-platform-tests.org/writing-tests/server-features.html talk about the {{expansions}} we do, and https://web-platform-tests.org/writing-tests/testharness-tutorial.html#writing-a-second-subtest mentions get_host_info().

I can't seem to find any guidelines about which to choose and when though. Is get_host_info() used mostly to get tests to run in WebKit's infrastructure (given #3544 for example)? Should more tests be using it in this case? Can someone with more knowledge document the properties that get_host_info() returns?

[marcoscaceres]

I think get_host_info() is still just using substitutions that are supported in WebKit's infrastructure. However, get_host_info() adds some indirection, which kinda sucks.

I personally think it's clearer putting the explicit URL... but of course, it's always annoying having to look up which URLs are supported by WebKit's infrastructure (sorry about that! Hopeful that will eventually be fixed).

[marcoscaceres]

(worst case scenario, just ping me to confirm the URLs... I can run the tests against WebKit!)

[marcoscaceres]

Hehe, I should rewrite get_host_info() and friends to be an ES6 module.