syscall: relax output check in TestGroupCleanupUserNamespace

syscall: relax output check in TestGroupCleanupUserNamespace

“If you have a procedure with ten parameters, you probably missed some.”
― attr. Alan J. Perlis

I argue that the same is true for hard-coded special cases.

In TestGroupCleanupUserNamespace, instead of a curated list of strings
observed in the wild we now check for a prefix, as was done for
TestGroupCleanup in CL 24670.

Updates #16224.
Updates #16303.
Updates #19938.
Updates #34547.
Updates #46752.
Fixes #52088.

Change-Id: I59c5b0c048113e306996c0f8247e09c714d2423a
Reviewed-on: https://go-review.googlesource.com/c/go/+/397316
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>