PHP8 libxml behaviour change: unwanted network requests (was: Slow LTI tests)

While working on MDL-73826 it was detected that mod/lti/tests/locallib.php unit tests are too slow (> 4 minutes). It seems that practically all the time is spent sort of "waiting" with this 4 tests (one minute each):

$ vendor/bin/phpunit mod/lti/tests/locallib_test.php --testdox -v

Moodle 3.11.5+ (Build: 20220208), 34b5b416393a5163a3273f298b2ab916ad126755

Php: 8.0.15, pgsql: 13.5, OS: Darwin 20.6.0 x86_64

PHPUnit 9.5.13 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.0.15

Configuration: /Users/stronk7/git_moodle/moodle/phpunit.xml

locallib_test (mod_lti\locallib_test)

...

...

...

 ✔ Lti get launch data default organizationid unset usehost [60410.68 ms]

 ✔ Lti get launch data default organizationid set usehost [60401.39 ms]

 ✔ Lti get launch data default organizationid set usesiteid [60440.08 ms]

 ✔ Lti get launch data default organizationid orgid override [60394.67 ms]

...

...

Ideally, we should be able to be in the ms range instead of 1 minute.

Updated description of the problem and the fix, after all the research (from commit message):

Summary:

• With PHP7, libxml_disable_entity_loader() was making those
  flags futile, and the DTDs were not being requested ever.
• With PHP8, the flags get precedence and requests for the
  DTD have started to happen.
• Those requests are not served by W3C (1 minute time-out).
• Those DTDs aren't used by anything in code (no entity
  replacement, no validation...)

Hence:

• Remove the flags so it will work the same in PHP7 and PHP8.
• Just to double ensure we don't want any remote loading
  to happen ever, add explicitly the LIBXML_NONET flag.

Ciao