Update Snapshots for MR Popovers

A new attribute was added to `GlPopover`, so the snapshots had to be
updated to match.

spec/frontend/mr_popover/__snapshots__/mr_popover_spec.js.snap

@@ -3,6 +3,7 @@
 exports[`MR Popover loaded state matches the snapshot 1`] = `
 <glpopover-stub
   boundary="viewport"
+  cssclasses=""
   placement="top"
   show=""
   target=""
@@ -61,6 +62,7 @@ exports[`MR Popover loaded state matches the snapshot 1`] = `
 exports[`MR Popover shows skeleton-loader while apollo is loading 1`] = `
 <glpopover-stub
   boundary="viewport"
+  cssclasses=""
   placement="top"
   show=""
   target=""