testing/synctest, runtime: avoid panic when using linker-alloc WG from bubble

testing/synctest, runtime: avoid panic when using linker-alloc WG from bubble

We associate WaitGroups with synctest bubbles by attaching a
special to the WaitGroup. It is not possible to attach a special
to a linker-allocated value, such as:

    var wg sync.WaitGroup

Avoid panicking when accessing a linker-allocated WaitGroup
from a bubble. We have no way to associate these WaitGroups
with a bubble, so just treat them as always unbubbled.
This is probably fine, since the WaitGroup was always
created outside the bubble in this case.

Fixes #74005

Change-Id: Ic71514b0b8d0cecd62e45cc929ffcbeb16f54a55
Reviewed-on: https://go-review.googlesource.com/c/go/+/679695
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>