testsuite: Rename isCross() predicate to needsTargetWrapper()

isCross() was a misnamed because it assumed that all cross targets would
provide a target wrapper, but the two most common cross targets
(javascript, wasm) don't need a target wrapper.

Therefore we rename this predicate to `needsTargetWrapper()` so
situations in the testsuite where we can check whether running
executables requires a target wrapper or not.