Prevent test environment contamination when HTTPS related assertions fail

[johnbillion]

Several tests which are related to HTTPS alter the $_SERVER['HTTPS'] variable, but then perform assertions before this value is reset. When these assertions fail, it has a knock-on affect for other tests because the resets don't occur, and we get cascading failures as a result.

Ideally we should audit every one of our tests to ensure any environment changes (including things such as post type registration, messing with other superglobals, etc) only perform their assertions after the resets happen, but the HTTPS tests are what I'm focusing on for now.

[johnbillion]

In 36711:

Move some assertions in HTTPS related tests, so failures that occur before the environment reset don't result in a contaminated test environment.

See #35954

[johnbillion]

In 36715:

Unify the initialisation of $_SERVER variables during test bootstrap. This abstracts the (re-)initialisation into a function that can be used inside of tests too, before assertions are performed.

See #35954

[johnbillion]

In 36721:

Remove (or at least reduce) the need to reset common $_SERVER variables before assertions or between tests, by introducing a method which automatically resets them during test setup.

See #35954

[johnbillion]

In 36722:

Refactor some janky URL tests into data providers for clarity and better error reporting.

See #35954