[tests-only] Test with postgres:9.4

[phil-davis]

to see if the problem fixed in #38682 causes any tests to fail in CI

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

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[phil-davis]

Everything passes with postgres:9.4

[phil-davis]

With postgres:9.6
https://drone.owncloud.com/owncloud/core/29759/151/15
webUIWebdavLocks/unlock.feature:205 fails. But that is already tagged @skipOnDbPgsql - we do not expect that to pass anyway, for different reasons:
"Pgsql and Oracle database can have different sorting, so allow this scenario to be skipped on those"

And https://drone.owncloud.com/owncloud/core/29759/84/13 apiTrashbin/trashbinFilesFoldersOc10Issue23151.feature:44 failed. That is issue #23151 and the bug-demo test is known to be not reliable on postgres and Oracle.

Everything else passes. So we do not seem to have a test scenario that exposes the bug that is being fixed in #38682