Strengthen some GUI tests

Author: GuillaumeGomez

tests/rustdoc-gui/methods-left-margin.goml

@@ -8,10 +8,12 @@ assert: ".impl-items > section.method"
 
 // Checking on desktop.
 set-window-size: (900, 600)
+wait-for-size: ("body", {"width": 900})
 store-position: (".impl-items section.method", {"x": x})
 assert-position: (".impl-items section.method", {"x": |x|}, ALL)
 
 // Checking on mobile.
 set-window-size: (600, 600)
+wait-for-size: ("body", {"width": 600})
 store-position: (".impl-items section.method", {"x": x})
 assert-position: (".impl-items section.method", {"x": |x|}, ALL)

tests/rustdoc-gui/notable-trait.goml

@@ -84,6 +84,7 @@ call-function: ("check-notable-tooltip-position", {
 
 // Checking on mobile now.
 set-window-size: (650, 600)
+wait-for-size: ("body", {"width": 650})
 call-function: ("check-notable-tooltip-position-complete", {
     "x": 25,
     "i_x": 303,