This updates TestGroupCleanup and TestGroupCleanupUserNamespace to pass in the
Alpine builder.

Updates #19938

Change-Id: Iacbfd73782eccd57f872f9e85726c6024529c277
Reviewed-on: https://go-review.googlesource.com/40692Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>