Fix some URLs whose sub-path is missing (#31289)

Fix #31285

templates/devtest/fetch-action.tmpl

@@ -25,7 +25,7 @@
 				<div><label><input name="check" type="checkbox"> check</label></div>
 				<div><button name="btn">submit post</button></div>
 			</form>
-			<form method="post" action="/no-such-uri" class="form-fetch-action">
+			<form method="post" action="no-such-uri" class="form-fetch-action">
 				<div class="tw-py-8">bad action url</div>
 				<div><button name="btn">submit test</button></div>
 			</form>