Add GUI regression test for setting's cog color

tests/rustdoc-gui/settings-button.goml

@@ -0,0 +1,31 @@
+// This test ensures that the icon of the settings button looks as expected on
+// all themes.
+include: "utils.goml"
+go-to: "file://" + |DOC_PATH| + "/test_docs/index.html"
+show-text: true
+
+define-function: (
+    "check-image",
+    [theme, filter],
+    block {
+        call-function: ("switch-theme", {"theme": |theme|})
+        assert-css: ("#settings-menu > a::before", {
+            "filter": |filter|,
+            "width": "22px",
+            "height": "22px",
+        })
+    }
+)
+
+call-function: ("check-image", {
+    "theme": "ayu",
+    "filter": "invert(1)",
+})
+call-function: ("check-image", {
+    "theme": "dark",
+    "filter": "none",
+})
+call-function: ("check-image", {
+    "theme": "light",
+    "filter": "none",
+})

tests/rustdoc-gui/settings.goml

@@ -1,6 +1,5 @@
 // This test ensures that the settings menu display is working as expected and that
 // the settings page is also rendered as expected.
-include: "utils.goml"
 go-to: "file://" + |DOC_PATH| + "/test_docs/index.html"
 show-text: true // needed when we check for colors below.
 // First, we check that the settings page doesn't exist.