How long the test is
normally expected to run before returning.
(String "eternal", "long", "moderate", or "short"
with the default derived from a test's size attribute)
While a test's size attribute controls resource estimation, a test's
timeout may be set independently. If not explicitly specified, the
timeout is based on the test's size (with "small" ⇒ "short",
"medium" ⇒ "moderate", etc...). "short" means 1 minute, "moderate"
5 minutes, and "long" 15 minutes.