[BUGFIX] Page tree unit test fails if pages don't exist

The unit test getNodesSetsIsMountPointField fails if
the pages with uid 1, 2, 3 are not available.

Instead of relying on user dependent data, we should mock
it. The patch introduces a wrapper method for getRecordWSOL
to be able to mock that method and changes the test to do
that.

Change-Id: I36ec23a47bd39821595e787fba1cb8b77e2c3b0c
Fixes: #38503
Releases: 6.0, 4.7
Reviewed-on: http://review.typo3.org/12478
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn