Ensure seeded listings are valid (#14784)

This matches the same change made to the db/seeds.rb file at
https://github.com/forem/forem/blob/main/db/seeds.rb#L515

Since there's a validation that Listings are only 12 lines or
shorter, and no technical limit on the length of
`Faker::Markdown.random`'s output, take the first 10 lines only.

spec/support/seeds/seeds_e2e.rb

@@ -465,7 +465,7 @@ seeder.create_if_none(Listing) do
   Listing.create!(
     user: admin_user,
     title: "Listing title",
-    body_markdown: Faker::Markdown.random,
+    body_markdown: Faker::Markdown.random.lines.take(10).join,
     location: Faker::Address.city,
     organization_id: admin_user.organizations.first&.id,
     listing_category_id: ListingCategory.first.id,