Support a Timeout similar to ntest

We just tripped over an unexpected interaction between `serial_test::serial` and `ntest::timeout` in that our tests were failing because ntest is measuring from the start of the invocation of the test, and not when serial actually acquires the mutex. I can see that this crate used to have this functionality, but it was removed in #83. I'm curious if there is any appetite for re-adding this feature and trying to work out correct semantics for measuring the execution time of the body of the test?