Run tests for stable10 on PHP 7.3

[phil-davis]

Unit tests:

1) OCA\DAV\Tests\unit\ServerTest::test with data set "principals" ('principals/users/admin', array('caldav', 'oc-resource-sharing', 'carddav'))
ErrorException: "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"?

/drone/src/lib/composer/sabre/dav/lib/CalDAV/ICSExportPlugin.php:321
/drone/src/apps/dav/lib/Server.php:160
/drone/src/apps/dav/tests/unit/ServerTest.php:45

Is fixed by sabre-io/dav#1080 which is in sabre/dav 4.0.0-alpha5

2) OCA\DAV\Tests\unit\CardDAV\CardDavBackendTest::testUpdateProperties
ErrorException: "continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"?

/drone/src/lib/composer/sabre/vobject/lib/Component/VCard.php:514
/drone/src/apps/dav/tests/unit/CardDAV/CardDavBackendTest.php:381

(8 of these)
Are fixed by sabre-io/vobject#424 which is in sabre/vobject 4.2.0-alpha1 which is a dependency for sabre/dav 4.0.0-alpha5
So that would need backporting to some version combination that will support PHP 5.6

[phil-davis]

Good to know the above things that currently prevent PHP 7.3 from passing in stable10
@DeepDiver1975 FYI, I guess at some point we will want to support PHP 7.3 with stable10 ?
And so these sabre changes will need to be released as patches in versions of sabre libraries that also are happy to co-exist with PHP5.6