Refactoring in commit 66419bc0 (CTest: convert timeouts to
std::chrono::duration, 2017-11-20) accidentally changed the logic used
to compute the timeout for a test when it starts.  It incorrectly limits
the maximum possible timeout to 2 minutes rather than 2 minutes less
than the total allowed test time remaining.  Update the new logic to
restore the original behavior.

Avoid subtracting 2 minutes from our "infinite" timeout value to avoid
creating very large timeouts that are not "infinite" and may exceed
integer type ranges.