fix jest's test.table flow type

Summary:
The current flow type disallows passing in an `Array<string>` since the current
type signature would allow just to append to the `Array`, `$ReadOnlyArray`
doesn't allow writing.

@public

Reviewed By: jstejada

Differential Revision: D16354977

fbshipit-source-id: a83c1227c1a15225487ac8672818d8b319dce32f