test262: add $DONE function to adapter

test262 uses a `$DONE` function for async tests with
semantics like those of mocha's `done`.

Briefly: done(arg) => if (arg) { /* failure */ }

Implemented a version of this for v8, using v8-specific
api (`print`, `quit`)
test262: clean up testcfg

originally several changes to support async tests.

turned out not to be necessary, so just some code cleanup

 - DRY setting up paths to test262 suite, harness, etc
 - remove redundant argument `harnesspath`
 - clean up helper fns