Fix baseline alignment of dropdown-button in Firefox.

A previous change added an overflow property to the dropdown-button so that dropdowns don't wrap. But the overflow property, along with inline-flex display and a bunch of other complicated conditions, triggers this bug in Firefox.

It looks like setting a max-width on the dropdown-button is enough to get the desired behavior for material-dropdown-select. So let's do that instead.

Also, add a Scuba test so this alignment stops breaking.

PiperOrigin-RevId: 176144876

Author: mgreenland

lib/material_select/dropdown_button.scss

@@ -12,8 +12,8 @@ $error-text-color: #d34336;
   display: inline-flex;
   flex: 1;
   flex-direction: column;
+  max-width: 100%;
   min-height: $icon-size;
-  overflow: hidden;
 }
 
 .button {