Test the export timeout by waiting indefinitely for the export to
timeout instead of having a second timer, in its own goroutine, timeout.
The algorithm this replaces fails on machines that are slow and the one
meta-timer is given priority to progress over the export timer that is
being testing, resulting in a false-negative test result.
Move the testing of a BatchSpanProcessor export timeout to its own test
function. This removes the bloat this introduces to the other testing
options and allows customization that enable the testing in a
deterministic manner.