Set expected value of line-height to expanded generic event list summary (GELS) spacer on IRC layout (#10211)

* Set expected line height to an expanded GELS line on IRC layout, add a test
* Add a test for compact modern/group layout
* Create a new test category on timeline.spec.ts
* Rename the class name as 'spacer'
* Add a test for GELS' spacer on bubble layout
---------

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>
Co-authored-by: Germain <germain@souquet.com>

res/css/views/rooms/_IRCLayout.pcss

@@ -20,8 +20,9 @@ $irc-line-height: $font-18px;
     --name-width: 80px; // cf. ircDisplayNameWidth on Settings.tsx
     --icon-width: 14px;
     --right-padding: 5px;
+    --line-height: $irc-line-height;
 
-    line-height: $irc-line-height !important;
+    line-height: var(--line-height) !important;
 
     .mx_NewRoomIntro {
         > h2 {