test(selftest): pin the top-tab underline invariant

TopTabBar had no observability at all -- no counter, no self-test reaching it.
It now exposes LastRenderedUnderlineCount and MainWindow hands the component
out the same way it already does for the sidebar.

Three cases: one of two tabs active draws exactly one underline, a null active
tab draws zero, and an active tab that is not in the list also draws zero. The
last one matters because the strip skips popped-out tabs, so the active tab
legitimately need not be among the drawn ones.

HellionChat/Plugin.cs

@@ -455,6 +455,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.CurrentTabCouplingStep(this),
                 new SelfTests.SidebarUnreadDotStep(this),
                 new SelfTests.SidebarActiveSurfaceStep(this),
+                new SelfTests.TopTabUnderlineStep(this),
                 new SelfTests.UnreadDecisionStep(),
                 new SelfTests.CurrentTabGuidedStep(this),
                 new SelfTests.CardClipPlanStep(this),