runtime: store zero-delay mutex contention events

Mutex contention events with delay of 0 need more than CL 604355 added:
When deciding which event to store in the M's single available slot,
always choose to drop the zero-delay event. Store an explicit flag for
whether we have an event to store, rather than relying on a non-zero
delay.

And, fix a test of sync.Mutex contention that expects those events to
have non-zero delay. The reporting of non-runtime contention like this
has long allowed zero-delay events, which we see when cputicks has low
resolution.

Fixes #68892
Fixes #68906

Change-Id: Id412141e4eb09724f3ce195899a20d59c92d7b78
Reviewed-on: https://go-review.googlesource.com/c/go/+/606115


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>