runtime: make TestDebugLogInterleaving much more robust
The current test often doesn't actually generate enough interleaving
to result in multiple log shards. This CL rewrites this test to
forcibly create at least 10 log shards with interleaved log messages.
It also tests dlog's robustness to being held across M and P switches.
Change-Id: Ia913b17c0392384ff679832047f359945669bb15
Reviewed-on: https://go-review.googlesource.com/c/go/+/600699
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Austin Clements <austin@google.com>