runtime: don't overwrite global stop channel in tests
This is used by TestStopTheWorldDeadlock, and the new
TestReadMetricsSched test accidentally modifies this global variable
instead of (as intended) defining a local one.
Change-Id: I7aaece83f285d051ad8b56b7591c76613c39613a
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/696556 Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>