fix: don't serialize listener calls in emit

`Executor#emit` waits for listeners to resolve, which allows Intern to
stay in sync with event processing code. However, calls to individual
listeners don't need to wait on each other. This can cause event
handlers to become interleaved (e.g., a `testStart` handler might be
called before a `testEnd` handler for the previous test had been
called.)

references #1054
resolves #1124