Rollup merge of #111844 - GuillaumeGomez:migrate-gui-test-color-4, r=notriddle

Migrate GUI colors test to original CSS color format

I updated the `browser-ui-test` version because I fixed GuillaumeGomez/browser-UI-test#507. If inside a function, the colors were not considered, preventing the second commit of this PR.

Follow-up of #111459.

r? `@notriddle`

Author: Dylan-DPC

src/ci/docker/host-x86_64/x86_64-gnu-tools/browser-ui-test.version

@@ -1 +1 @@
-0.16.3
+0.16.4

tests/rustdoc-gui/scrape-examples-color.goml

@@ -81,16 +81,16 @@ define-function: (
 
 call-function: ("check-background", {
     "theme": "ayu",
-    "background_color_start": "rgb(15, 20, 25)",
+    "background_color_start": "rgba(15, 20, 25, 1)",
     "background_color_end": "rgba(15, 20, 25, 0)",
 })
 call-function: ("check-background", {
     "theme": "dark",
-    "background_color_start": "rgb(53, 53, 53)",
+    "background_color_start": "rgba(53, 53, 53, 1)",
     "background_color_end": "rgba(53, 53, 53, 0)",
 })
 call-function: ("check-background", {
     "theme": "light",
-    "background_color_start": "rgb(255, 255, 255)",
+    "background_color_start": "rgba(255, 255, 255, 1)",
     "background_color_end": "rgba(255, 255, 255, 0)",
 })