test262.py: only include helper scripts when needed

test262.py: only supply async helper scripts when test is async
sth.js: factor out function isAsyncTest()
timer.js: improve workaround for async tests when Promise is defined but setTimeout is noot

timer.js emulates setTimeout using Promise by doing a busy loop that checks
if `timeout` milliseconds have elapsed.  Modified check to (timeLeft > 0) instead
of (!timeLeft) to prevent infinite loop when check does not happen to run
at precise millisecond timeout expires.

Because test262.py did not support the $INCLUDE directive, some helper
scripts were added to every test -- notably testIntl, timer, and donePrintHandle
Now that $INCLUDE is supported, these can be dropped, speeding overall test run time
Merge pull request #49 from smikes/faster-python-runner

test262.py: only include helper scripts when needed