Fix test_popupwin failure due to syntax highlight change #1012

Job	Run time
build-and-test (macos-15, 16.4, true, true, true) / build-and-test	25m 57s
build-and-test (macos-14, 15.4, false) / build-and-test	9m 48s
build-and-test (macos-13, 15.2, true, vimtags, check-xcodeproj-compat) / build-and-test	21m 9s
build-and-test (macos-13-xcode14-self-hosted, true, _10.9, true, true) / build-and-test	0s
	56m 54s

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