Mark some more flaky tests as flaky

I really want end2end tests to die...

tests/end2end/features/backforward.feature

@@ -74,6 +74,7 @@ Feature: Going back and forward.
                   url: http://localhost:*/data/backforward/1.txt
                 - url: http://localhost:*/data/backforward/2.txt
 
+    @flaky
     Scenario: Going back with count.
         Given I open data/backforward/1.txt
         When I open data/backforward/2.txt

tests/end2end/features/history.feature

@@ -104,7 +104,7 @@ Feature: Page history
         Then the page should contain the plaintext "4.txt"
 
     # Hangs a lot on AppVeyor
-    @posix
+    @posix @flaky
     Scenario: Listing history with qute:history redirect
         When I open data/numbers/3.txt
         And I open data/numbers/4.txt

tests/end2end/test_hints_html.py

@@ -117,6 +117,7 @@ def test_hints(test_name, zoom_text_only, zoom_level, find_implementation,
         quteproc.set_setting('hints.find_implementation', 'javascript')
 
 
+@pytest.mark.flaky
 def test_word_hints_issue1393(quteproc, tmpdir):
     dict_file = tmpdir / 'dict'
     dict_file.write(textwrap.dedent("""