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.