"[The modal particle] einmal, shortened mal (literally: once, translation roughly: "for once") also indicates a certain immediacy to the action or even implies a command. On the other hand, it can give a kind of casualness to a sentence and so making it sound less blunt."
quis_lib_duo touched on this a bit, but yeah, in this case, I'd say it sorta softens up the command into a less direct one. "Testen wir" -- We're testing. "Testen wir mal" -- Let's test (now).
What is wrong with "Let us test how fast does he run" - it was not accepted
Why is there a comma in this sentence? In English there shouldn't be, but is there a different German grammar rule for commas?
Yes, there is. „Testen wir mal“ is the main clause here, „wie schnell er läuft“ is a subordinate clause. They have to be separated by a comma.