tests: Ignore "SelectionRequest too old" globally.

If we use pytestmark, it won't be ignored inside subprocess output, so our fix
from https://github.com/The-Compiler/qutebrowser/issues/1124#issuecomment-158073581
never actually worked.

See #1124 and #1183.

pytest.ini

@@ -38,3 +38,4 @@ qt_log_ignore =
     ^QWaitCondition: Destroyed while threads are still waiting
     ^QXcbXSettings::QXcbXSettings\(QXcbScreen\*\) Failed to get selection owner for XSETTINGS_S atom
     ^QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to .*
+    ^QXcbClipboard: SelectionRequest too old

tests/integration/features/test_caret.py

@@ -26,9 +26,4 @@ from PyQt5.QtGui import QClipboard
 from test_yankpaste import skip_with_broken_clipboard
 
 
-# https://github.com/The-Compiler/qutebrowser/issues/1124#issuecomment-158073581
-pytestmark = pytest.mark.qt_log_ignore(
-    '^QXcbClipboard: SelectionRequest too old', extend=True)
-
-
 bdd.scenarios('caret.feature')

tests/integration/features/test_yankpaste.py

@@ -24,9 +24,6 @@ import pytest_bdd as bdd
 
 bdd.scenarios('yankpaste.feature')
 
-# https://github.com/The-Compiler/qutebrowser/issues/1124#issuecomment-158073581
-pytestmark = pytest.mark.qt_log_ignore(
-    '^QXcbClipboard: SelectionRequest too old', extend=True)
 
 @pytest.fixture(autouse=True)
 def skip_with_broken_clipboard(qtbot, qapp):