That's what this class really is; in fact that's what the first line of
the comment says it is.
This commit does not rename the main files, since those will contain
other time-related classes in a little bit.
Simplify a lot of uses of ElapsedTimer by converting the callers to
elapsed_time from elapsed, as the AK::Time returned is better for unit
conversions and comparisons against constants.
This is a regression test to validate the functionality that was
reported broken in #9071, where the kernel would spin attempting
to cancel a stale timer.
