Apparently, at least on Mac, there are circumstances in which the very-large-
message test can take several times longer than normal, yet still complete
successfully. This is always the problem with timeouts: does timeout
expiration mean that the code in question is actually hung, or would it
complete if given a bit longer?
If very-large-message test fails, retry a few times with smaller sizes to try
to find a size at which the test runs reliably. The default size, ca 1MB, is
intended to be substantially larger than anything we'll encounter in the wild.
Is that "unreasonably" large? Is there a "reasonable" size at which the test
could consistently pass? Is that "reasonable" size still larger than what we
expect to encounter in practice? Need more information, hence this code.