runtime: make concurrent map test less flaky

Pre-populate the map, so no actual allocations occur during
map updates. That lessens the chance of heap corruption.

Also get rid of the want2 cases in the test. Those cases are no
longer possible after CL 770303.

Update #79671
Maybe fixes, if it avoids an infinite loop situation. Not sure
how that would happen, but maybe?

Change-Id: Id764184584f2312fe59b2091c4addd90d55c56a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/783360
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>