When reading from time.Timer.C for an expired timer using
a fake clock (in a synctest bubble), the timer will not
be in a heap. Avoid a spurious panic claiming the timer
moved between synctest bubbles.

Drop the panic when a bubbled goroutine reads from a
non-bubbled timer channel: We allow bubbled goroutines
to access non-bubbled channels in general.

Fixes #70741

Change-Id: I27005e46f4d0067cc6846d234d22766d2e05d163
Reviewed-on: https://go-review.googlesource.com/c/go/+/634955


Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>