v0.44.1: Preferences headings stay bold under the app's style sheet

The headings were set with setFont, which the app's own style sheet
(theme.apply_theme) overrides on re-polish — bold in a bare dialog, plain
in the running app. A style sheet on the heading itself wins. The test now
applies the theme first, as the app does.

tests/test_round79.py

@@ -161,14 +161,19 @@ def _prefs_dialog(prefs):
 
 class TestHeadings:
     def test_bold_and_two_px_larger(self, qapp, tmp_path):
-        dialog = _prefs_dialog(Preferences(tmp_path))
+        from lintunes import theme
+        prefs = Preferences(tmp_path)
+        theme.apply_theme(QApplication.instance(), prefs)   # as the app runs
+        dialog = _prefs_dialog(prefs)
+        dialog.show()
         text_px = QFontInfo(QApplication.font()).pixelSize()
         headings = dialog.findChildren(QLabel, "prefs-heading")
         assert {h.text() for h in headings} >= {"Interface", "Visualizer",
                                                  "Colors", "Last.fm"}
         for heading in headings:
             assert heading.font().bold()
-            assert heading.font().pixelSize() == text_px + 2
+            assert QFontInfo(heading.font()).pixelSize() == text_px + 2
+        dialog.close()
 
     def test_the_boxes_contents_stay_plain(self, qapp, tmp_path):
         dialog = _prefs_dialog(Preferences(tmp_path))