test(all): inject test styles on flag

core/scripts/screenshot/testing.css

@@ -1,3 +0,0 @@
-:root {
-  --ion-font-family: Arial;
-}

core/scripts/screenshot/testing.js

@@ -0,0 +1,15 @@
+{
+  const getStyles = () => {
+    const tag = document.createElement('style');
+    tag.textContent = `
+      :root {
+        --ion-font-family: Arial;
+      }
+    `;
+    return tag;
+  };
+
+  if (window.location.search.indexOf('ionic:_testing=true') > -1) {
+    document.head.appendChild(getStyles());
+  }
+}