Unit tests: Run `'bp_template_redirect'` hook in `go_to()` method

[r-a-y]

While looking at conditional loading in #7218, I ran into some issues with how we use the go_to() method to mock a visit to a page in PHPUnit.

In the go_to() method, we only run 'bp_init', which gets us halfway there. We also need to run 'bp_template_redirect' so hooks running on 'bp_actions' or 'bp_screens' can also run.

I'm guessing the issue why we never ran 'bp_template_redirect' is due to the use of wp_redirect(), which causes "headers are already sent" notices when running PHPUnit.

Attached patch alters bp_core_redirect() to only redirect if PHPUnit isn't running.

This also allows us to remove some hacky attempts at running certain hooks in our tests, most notably in @group mentions. This was the main problem that led me to fix this!

Let me know if you have any questions.

[r-a-y]

Sorry @netweb! BP Trac auto-assigned you to this issue because you're the Build/Test Tools maintainer.

[DJPaul]

let's give it a go. I remember how much of a pain it was to get go_to() "working" originally so not surprised that it's sort of broken. :)

[r-a-y]

In 11882:

Unit Tests: Run 'bp_template_redirect' hook in go_to() method.

In PHPUnit, we mock page visits with the go_to() method, however we
previously only ran the 'bp_init' hook, which sets up various internal
page routing properties, but doesn't run any screen hooks.

This commit adds the 'bp_template_redirect' hook to the go_to() method,
which allows hooks like 'bp_actions' and 'bp_screens' to run. In order
for those hooks to run without errors in PHPUnit, we need to adjust our
bp_core_redirect() function so it doesn't kill execution. Also, for
PHPUnit, we do not allow redirects to occur since they cause 'headers
already sent' notices.

Lastly, we can adjust some unit tests to just use the go_to() method,
instead of needing to manually call a screen hook.

Fixes #7703.