Add a test that shows heading nav conflict with search mark

The search marker is getting copied into the sidebar, but it cannot be
dismissed.

Author: ehuss

tests/gui/books/heading-nav/src/SUMMARY.md

@@ -7,3 +7,4 @@
 - [Headings with markup](markup.md)
 - [Current scrolls to bottom](current-to-bottom.md)
 - [Unusual heading levels](unusual-heading-levels.md)
+- [Filtered headings](filtered-headings.md)

tests/gui/books/heading-nav/src/filtered-headings.md

@@ -0,0 +1,5 @@
+# Filtered headings
+
+## Skateboard
+
+Checking for search marking.

tests/gui/heading-nav-filter.goml

@@ -0,0 +1,6 @@
+// Tests for collapsed heading sidebar navigation.
+
+set-window-size: (1400, 800)
+go-to: |DOC_PATH| + "heading-nav/filtered-headings.html?highlight=skateboard#skateboard"
+
+assert-property: ("//a[contains(@class, 'header-in-summary') and @href='#skateboard']", {"innerHTML": '<mark data-markjs="true">Skateboard</mark>'})