Support MediaWiki PHP tests under HHVM

Experimental replacement of mediawiki-phpunit with two new jobs:

    mediawiki-phpunit-hhvm  (hhvm from Trusty)
    mediawiki-phpunit-zend  (PHP 5.3 from Precise)

Same for all extensions!

Depend on puppet change https://gerrit.wikimedia.org/r/178806/ to setup
hhvm on the CI slave with a reasonable configuration (already applied on
the contint slaves).

Add an assertion macro to make sure we use the proper PHP flavor. Pass
it either 'hhvm' or 'zend' and it will parse 'php --version', aborting
on mismatch.

Add a macro to delete '*.hhbc' files. They are bytecodes compiler cache
files generated by hhvm.  Set via the HHVM_REPO_CENTRAL_PATH env
variable.

Convert the 'mediawiki-phpunit' job template to vary by a new variable
'phpflavor'. The variable is used to select a proper node using a crazy
label expression. CI slaves labels have been adjusted already with
either 'phpflavor-hhvm' or 'phpflavor-zend'.

Convert the testextension jobs as well.

Do not run the hhvm MediaWiki jobs on REL1_23 and REL1_24.

Add the new jobs to the experimental pipeline of mediawiki/core so
people can manually trigger them from Gerrit by commenting:
'check experimental'.

Keep the old jobs around since they are still triggered by the test and
gate-and-submit pipelines.

Do not run the mediawiki hhvm job on REL1_23/REL1_24 branch, leaving it
open for the future REL1_25 branch though.

TODO:
* migrate the qunit jobs
* Fix hhvm test ApiFormatWddx (T75531). That is a bug upstrem which
  would be fixed with a new hhvm version scheduled to be deployed next
  week (-wm6).

Bug: T75521
Change-Id: I924a42d2001edafa5ab00623b342afd87cfa5334