Fix ms_subdomain_constants()

[TobiasBg]

ms_subdomain_constants() brings parity between the SUBDOMAIN_INSTALL and the deprecated VHOST constants, introduced in [14452].

It's a two-step process, where the first run of the function checks and sets the constants as necessary, and where the second run is used to print debug warnings/errors.

However, the function does not account for "internal success". If the VHOST constant was not set (it shouldn't be anymore), it doesn't set $error=false; so that all checks are unnecessarily performed again in the next evaluation of the function (the if-condition false === $error further up in the function is never true).
(If one then runs the function for a third time, an error will be shown, although nothing was wrong initially.)

Also, one of the if-conditions can be simplified.

[wonderboymusic]

In 29200:

Properly set $subdomain_error to false when applicable in ms_subdomain_constants(). This was previously untestable because it used static vars: use globals instead.

Adds unit test.
Fixes #28697.

[wonderboymusic]

Props TobiasBg.

[SergeyBiryukov]

I don't think the unit test here is important enough to replace static variables with globals.

[wonderboymusic]

In 29250:

After [29200], switch back to using static vars instead of adding 2 globals, as per Sergey.

Fixes #28697.