Fix UI tests (jupyterlab#277)

* Upgrade galata to lab 4.1

Fix lockfile

* Use `jp-button` instead of button

* Be more explicit with test checking for "user menu with 2 items"

(it is not!); because in lab 4.1 the DOM hierarchy changed (toolbar
is now using shadow DOM), instead select the things which are of
interest using precise selectors

* Update snapshots

* Update two more snapshots, switch serial assertions to soft

* Fix `Create a notebook` test

Author: krassowski

ui-tests/yarn.lock

@@ -2083,7 +2083,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"csstype@npm:3.0.10":
+"csstype@npm:3.0.10, csstype@npm:^3.0.2":
   version: 3.0.10
   resolution: "csstype@npm:3.0.10"
   checksum: 20a8fa324f2b33ddf94aa7507d1b6ab3daa6f3cc308888dc50126585d7952f2471de69b2dbe0635d1fdc31223fef8e070842691877e725caf456e2378685a631