tests: Skip pdf.js disabled test on QtWebKit with PDF image plugin

Since Qt 5.15, QtWebEngine comes with Qt PDF as TP:
https://wiki.qt.io/New_Features_in_Qt_5.15#Technology_Preview_Modules

This also includes a new PDF image plugin in
/usr/lib/qt/plugins/imageformats/libqpdf.so. Due to that, QtWebKit now displays the
PDF's first page as image rather than triggering a download.

See https://github.com/qtwebkit/qtwebkit/issues/1013

Thus, detect this condition (both QtWebKit and QtWebEngine installed) and skip the test
in that scenario.

pytest.ini

@@ -38,6 +38,7 @@ markers =
     unicode_locale: Tests which need an unicode locale to work
     qtwebkit6021_xfail: Tests which would fail on WebKit version 602.1
     js_headers: Sets JS headers dynamically on QtWebEngine (unsupported on some versions)
+    qtwebkit_pdf_imageformat_skip: Broken on QtWebKit with PDF image format plugin installed
 qt_log_level_fail = WARNING
 qt_log_ignore =
     ^SpellCheck: .*

tests/end2end/conftest.py

@@ -28,9 +28,10 @@ import sys
 import shutil
 import pstats
 import operator
+import pathlib
 
 import pytest
-from PyQt5.QtCore import PYQT_VERSION
+from PyQt5.QtCore import PYQT_VERSION, QCoreApplication
 
 pytest.register_assert_rewrite('end2end.fixtures')
 
@@ -142,6 +143,9 @@ def pytest_collection_modifyitems(config, items):
     header_bug_fixed = (not qtutils.version_check('5.12', compiled=False) or
                         qtutils.version_check('5.15', compiled=False))
 
+    lib_path = pathlib.Path(QCoreApplication.libraryPaths()[0])
+    qpdf_image_plugin = lib_path / 'imageformats' / 'libqpdf.so'
+
     markers = [
         ('qtwebengine_todo', 'QtWebEngine TODO', pytest.mark.xfail,
          config.webengine),
@@ -160,6 +164,10 @@ def pytest_collection_modifyitems(config, items):
         ('js_headers', 'Sets headers dynamically via JS',
          pytest.mark.skipif,
          config.webengine and not header_bug_fixed),
+        ('qtwebkit_pdf_imageformat_skip',
+         'Skipped with QtWebKit if PDF image plugin is available',
+         pytest.mark.skipif,
+         not config.webengine and qpdf_image_plugin.exists()),
     ]
 
     for item in items:

tests/end2end/features/qutescheme.feature

@@ -177,6 +177,7 @@ Feature: Special qute:// pages
         And I open data/misc/test.pdf without waiting
         Then the javascript message "PDF * [*] (PDF.js: *)" should be logged
 
+    @qtwebkit_pdf_imageformat_skip
     Scenario: pdfjs is not used when disabled
         When I set content.pdfjs to false
         And I set downloads.location.prompt to false