test: improved describe/it names

Fix few test names to improve the output of `mocha -R spec`.
Fixed timing-dependent unit-tests failures

DebuggerAgent.resume unit-test should pass `done` as the callback
function, otherwise other tests may run before this one is finished.

Fixed launcher so that it runs the test callback only once, instead of
everytime a `break` event is emitted.
Fixed DebuggerClient.resume acting like stepInto

Fixed the code adding maxStringLength to V8 requests, so that it
no longer creates a new args object when the caller did not provide
one.