Make unknown initial page fail the test

It could be that the initial page is of unknown type.  Typically, because the wrong URL was
provided or webadmin hasn't started up yet.  Before this triggered a stack
trace.  With this patch, the corresponding test fails with a meaningful
error message.
Fix counting error in the tests

The wrong number of successful tests was reported if a non-zero
number of tests are ignored.
Add support for restricting the tests to run.

This allows a subset of tests to be specified.
Add more tests for CellAdmin.

NB.  The assumeThat causes the test to always succeed for HtmlUnitDriver.
This is not the intended behaviour; we want the test to be ignored.
Switch to Java 8, upgrade JUnit version