testlib: Skip test function if init() fails.

testlib: Skip test function if init() fails.

Prior to this commit, the following statement in the qtestlib
documentation was untrue: "If init() fails, the following testfunction
will not be executed, the test will proceed to the next testfunction."
If init() called QSKIP, the test function would be skipped, but if
init() reported a failure, the test function would still be executed
(even though doing so could be unsafe).

This commit makes testlib skip a test function if init() reports a
failure and enhances the selftests to cover skips and fails in both
init() and cleanup().

Task-number: QTBUG-20371
Change-Id: Id1cc8464ae0b8c257ae1b74dbe9189a501f5366b
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>