linkcheck: Reduce test duration on Windows (#14133)

tests/roots/test-linkcheck-case-check/conf.py

@@ -1 +1 @@
-# Empty config for linkcheck case sensitivity tests
+linkcheck_timeout = 0.25

tests/roots/test-root/conf.py

@@ -35,6 +35,7 @@ keep_warnings = True
 pygments_style = 'sphinx'
 show_authors = True
 numfig = True
+linkcheck_timeout = 0.25
 
 html_sidebars = {
     '**': [