runtime: don't spin looking for a tiny alloc address with asan or race
CL 674655 modified the checkfinalizers test to spin looking for an
appropriate address to trip the detector, but this doesn't work with
ASAN or in race mode, which both disable the tiny allocator.
Fixes #73834.
Change-Id: I27416da1f29cd953271698551e9ce9724484c683
Reviewed-on: https://go-review.googlesource.com/c/go/+/675395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>