Fix Fomantic UI dropdown icon bug when there is a search input in menu (#27225) (#27228)

Backport #27225 by @wxiaoguang

Fix #27224

And add the case to the devtest page.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/devtest/gitea-ui.tmpl

@@ -171,6 +171,7 @@
 				<span class="text">simple</span>
 				{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 				<div class="menu">
+					<div class="ui icon search input"><i class="icon">{{svg "octicon-search"}}</i><input type="text" value="search input in menu"></div>
 					<div class="item">item</div>
 				</div>
 			</div>