iRules when pool member is down

Syntactically you can also use nested if statements:

when HTTP_REQUEST { 
  if { [active_members My_APACHE_SAT_8443_Pool] < 1 } {
     HTTP::redirect http://example-test.com/ 
  } else {
    if { [HTTP::uri] eq "/" } {
     HTTP::redirect https://[HTTP::host]/help/html 
    }
  }
}

Don't know the actual implementation on F5 TCL, but the former might be a bit more compact and use less CPU cycles. If needed you can add a final else, too. If you need more redirects based on the path, you might use switch statement instead of nested if.