chore: use dataTestId prop

This is a hotfix and should be undone as soon as possible. Relates back
to the issue around 'data-testid' prop not getting passed down to
component.

src/components/simple/simple.vue

@@ -1,13 +1,13 @@
 <template>
   <div>
     <h2>Simple</h2>
-    <ix-img src="examples/pione.jpg" sizes="20vw" data-testid="simple-fluid" />
+    <ix-img src="examples/pione.jpg" sizes="20vw" dataTestId="simple-fluid" />
     <h2>Fixed</h2>
     <ix-img
       src="examples/pione.jpg"
       width="100"
       fixed
-      data-testid="simple-fixed-width"
+      dataTestId="simple-fixed-width"
     />
     <h2>Picture</h2>
     <div data-testid="simple-picture">