Repeated spurious rustdoc test failure: rustdoc-gui/help-page

[RalfJung]

I've seen this fail in two PRs today, e.g. here

  /checkout/tests/rustdoc-gui/help-page.goml help-page... FAILED
  Error:  `tests/rustdoc-gui/utils.goml` line 16
      from `tests/rustdoc-gui/help-page.goml` line 23
      from `tests/rustdoc-gui/help-page.goml` line 32: Error: Evaluation failed: The following errors happened: [localStorage item "rustdoc-theme" (of value "ayu") != "null"]: for command `assert-local-storage: {"rustdoc-theme": |theme|}`

Cc @rust-lang/rustdoc

[GuillaumeGomez]

Taking a look.