Context manager to disable logging handler

[nicoddemus]

Add a context manager to qtlog fixture that disables the custom handler:

def test_foo(qtlog):
    with qtlog.disabled():
         # in this block the original qInstallMsgHandler function is active

Also implement a mark which does the same thing but for the entire test:

@pytest.mark.no_qt_log
def test_foo(qtlog):
     # in this test the original qInstallMsgHandler function is active

(Feature idea from @The-Compiler in #54).