Remove QTEST_NOOP_MAIN from qtestlib API.

Remove QTEST_NOOP_MAIN from qtestlib API.

This macro is no longer used in Qt's tests and encourages writing tests
in a way that makes test reporting less accurate -- remove it to prevent
further misuse.  If a test can be determined at compile-time to be
inapplicable, it should be omitted from the build via .pro file logic.
If that is not possible (e.g. there is no suitable qmake variable), the
test's initTestCase() function should call QSKIP to skip the entire test
with a meaningful explanation.

Task-number: QTBUG-21851
Change-Id: Icacc8c5567a700191b6ef3fa94ee52ede94c5b34
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>