Fix tst_qwebengineaccessibility API test

Implement a custom QTEST_MAIN preprocessor macro in order to
support hardcoded Chromium content switches in the API tests.

Force renderer accessibility to be on instead of enabling it on
demand when a screen reader is detected.

Change-Id: I9600ac4b26f4656104ce27ff19accbed4ea817c6
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>