bugfix: setgid tests fail when primary group is unknown (#34729)

On systems with remote groups, the primary user group may be remote and may not exist on
the local system (i.e., it might just be a number). On the CLI, it looks like this:

```console
> touch foo
> l foo
-rw-r--r-- 1 gamblin2 57095 0 Dec 29 22:24 foo
> chmod 2000 foo
chmod: changing permissions of 'foo': Operation not permitted
```

Here, the local machine doesn't know about per-user groups, so they appear as gids in
`ls` output. `57095` is also `gamblin2`'s uid, but the local machine doesn't know that
`gamblin2` is in the `57095` group.

Unfortunately, it seems that Python's `os.chmod()` just fails silently, setting
permissions to `0o0000` instead of `0o2000`. We can avoid this by ensuring that the file
has a group the user is known to be a member of.

- [x] Add `ensure_known_group()` in the permissions tests.
- [x] Call `ensure_known_group()` on tempfile in `test_chmod_real_entries_ignores_suid_sgid`.

lib/spack/spack/test/permissions.py

@@ -16,6 +16,18 @@
 pytestmark = pytest.mark.skipif(sys.platform == "win32", reason="chmod unsupported on Windows")
 
 
+def ensure_known_group(path):
+    """Ensure that the group of a file is one that's actually in our group list.
+
+    On systems with remote groups, the primary user group may be remote and may not
+    exist on the local system (i.e., it might just be a number). Trying to use chmod to
+    setgid can fail silently in situations like this.
+    """
+    uid = os.getuid()
+    gid = fs.group_ids(uid)[0]
+    os.chown(path, uid, gid)
+
+
 def test_chmod_real_entries_ignores_suid_sgid(tmpdir):
     path = str(tmpdir.join("file").ensure())
     mode = stat.S_ISUID | stat.S_ISGID | stat.S_ISVTX
@@ -50,6 +62,8 @@ def test_chmod_rejects_world_writable_suid(tmpdir):
 
 def test_chmod_rejects_world_writable_sgid(tmpdir):
     path = str(tmpdir.join("file").ensure())
+    ensure_known_group(path)
+
     mode = stat.S_ISGID
     fs.chmod_x(path, mode)