syscall: some containers may fail syscall.TestSetuidEtc

The test previously had the hardcoded assumption that /proc/self/status
files had "Groups:" lines containing numerical IDs in ascending order.
Because of the possibility of non-monotonic ordering of GIDs in user
namespaces, this assumption was not universally true for all
/proc/self/gid_map setups.

To ensure this test can pass in those setups, sanity check failed
"Groups:" line matches with a string sorted version of the expected
values. (For the test cases here, numerical and string sorted order
are guaranteed to match.)

Fixes #46145

Change-Id: Ia060e80b123604bc394a15c02582fc406f944d36
Reviewed-on: https://go-review.googlesource.com/c/go/+/319591


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>