Consider adding an ESLint rule for no async in describe blocks

[ernestognw]

From @fvictorio:

I forgot to mention:

• The wrong async keywords in describe functions can be prevented with this rule.
• The missing await keywords are harder. Ideally you'd use typescript and the no-floating-promises rule, but I understand that that would be a huge undertaking.

Originally posted by @fvictorio in #4942 (comment)

We're considering adding only the ESLint rule and not migrating to Typescript to catch floating promises. However, we're open to alternatives.