internal/godebug: make test godebug calls not vary

internal/godebug: make test godebug calls not vary

I think there may be an issue in bisect search with
change set elements not actually being independent,
to be explored later.  For now, modify the test to
remove that property.

Change-Id: I4b171bc024795d950cf4663374ad1dfc4e2952fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/494036
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>