Enforce etags as strings

[PVince81]

To prevent this error happening in unit tests and also other potential issues, please force etags to be strings:

Test\Files\Cache\Updater::testTouch
Failed asserting that '530e215543792' is not equal to <string:530e215526934>.

/var/lib/jenkins/jobs/pull-request-analyser/workspace@2/tests/lib/files/cache/updater.php:245

As discussed last week, PHP tries to be clever and interprets that etag as a number with exponent "e".

@icewind1991 can you take care of this ?
Thanks.

[PVince81]

CC @MorrisJobke @butonic

[butonic]

Fixed with #7438. The problem is not that the etag has been converted to a number but that assertEqual cannot compare strings like this. AssertSame seems to work.