Flaky GUI tests with heading-nav

The following errors happened on CI:

..FF..........                                    (15/15)

======== tests/gui/heading-nav-markup.goml ========

Error:  `tests/gui/heading-nav-markup.goml` line 8: The following errors happened: [`Heading with html` isn't equal to `Heading with code or italic or bold or strike`]: for command `assert-text: (".current-header", "Heading with code or italic or bold or strike")`
Error:  `tests/gui/heading-nav-markup.goml` line 9: The following errors happened (for selector `.current-header`): [expected `Heading with <code>code</code> or <em>italic</em> or <strong>bold</strong> or <del>strike</del>` for property `"innerHTML"`, found `Heading with <span>html</span>`]: for command `assert-property: (".current-header", {"innerHTML": "Heading with <code>code</code> or <em>italic</em> or <strong>bold</strong> or <del>strike</del>"})`

======== tests/gui/heading-nav-normal-intro.goml ========

Error:  `tests/gui/heading-nav-normal-intro.goml` line 8: The following errors happened: [`And a sub heading` isn't equal to `The first heading`]: for command `assert-text: (".current-header", "The first heading")`


<= doc-ui tests done: 13 succeeded, 2 failed, 0 filtered out

There's likely some race where the test isn't properly waiting or something, I'm not sure.

Happened in https://github.com/rust-lang/mdBook/pull/2891

Start by running tests/gui/heading-nav-markup.goml and tests/gui/heading-nav-normal-intro.goml, focusing on the assertions at lines 8–9 and the reported heading-navigation failures. Reproduce the CI behavior and inspect whether those assertions wait for navigation and rendered markup to settle. Done means both tests pass reliably, including their text and innerHTML assertions.