groups: fixes to pass GNU's Testsuite

[jhscheer]

This passes the relevant tests for groups from GNU's Testsuite that currently fail on master branch.

============================================================================
Testsuite summary for GNU coreutils 8.32.162-4eda
============================================================================
PASS: tests/misc/groups-dash.sh
PASS: tests/misc/groups-process-all.sh

• add new feature to support multiple users (introduced in coreutils 8.31)
• sync help text with GNU's groups manpage
• add tests for everything

[rivy]

Some nits.

And thanks for tackling this issue!

[jhscheer]

@rivy Thanks for your feedback and doing the review.

[sylvestre]

Clippy isn't happy

error[E0425]: cannot find value `VERSION_MULTIPLE_USERS` in this scope
  --> /home/runner/work/coreutils/coreutils/tests/by-util/test_groups.rs:81:66
   |
12 | const VERSION_MIN_MULTIPLE_USERS: &str = "8.31"; // this feature was introduced in GNU's coreutils 8.31
   | ------------------------------------------------ similarly named constant `VERSION_MIN_MULTIPLE_USERS` defined here
...
81 |     let version_check_string = check_coreutil_version(util_name, VERSION_MULTIPLE_USERS);
   |                                                                  ^^^^^^^^^^^^^^^^^^^^^^
   |
help: a constant with a similar name exists
   |
81 |     let version_check_string = check_coreutil_version(util_name, VERSION_MIN_MULTIPLE_USERS);
   |                                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
help: consider importing this constant
   |
1  | use crate::test_id::VERSION_MULTIPLE_USERS;
   |

[sylvestre]

Cool
Move from
GNU tests summary = TOTAL: 601 / PASS: 169 / FAIL: 321 / ERROR: 25
to
GNU tests summary = TOTAL: 601 / PASS: 171 / FAIL: 319 / ERROR: 25

[jhscheer]

@sylvestre #2361 also brought two new PASSES, but those were way harder to achieve.

[rivy]

If you don't mind, rebase this off of #2452 when it's merged and the spelling errors will be fixed.

Thanks for the work!