tests: Replace os.getlogin by getpass.getuser

The test `test_run_cwd_in_combination_with_runas` fails in some test
environments (like Debian autopkgtest):

```
======================================================================
ERROR: test_run_cwd_in_combination_with_runas (unit.modules.test_cmdmod.CMDMODTestCase)
[CPU:0.0%|MEM:6.5%] cmd.run executes command in the cwd directory
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/unit/modules/test_cmdmod.py", line 449, in test_run_cwd_in_combination_with_runas
    runas = os.getlogin()
OSError: [Errno 6] No such device or address

----------------------------------------------------------------------
```

Therefore replace `os.getlogin` by `getpass.getuser` as recommended by
upstream: "For most purposes, it is more useful to use
`getpass.getuser()` since the latter checks the environment variables
`LOGNAME` or `USERNAME` to find out who the user is, and falls back to
`pwd.getpwuid(os.getuid())[0]` to get the login name of the current real
user id."

Bug-Debian: https://bugs.debian.org/964270
Signed-off-by: Benjamin Drung <benjamin.drung@ionos.com>

tests/pytests/unit/modules/test_cmdmod.py

@@ -5,6 +5,7 @@
 """
 
 import builtins
+import getpass
 import logging
 import os
 import re
@@ -449,7 +450,7 @@ def test_run_cwd_in_combination_with_runas():
     """
     cmd = "pwd"
     cwd = "/tmp"
-    runas = os.getlogin()
+    runas = getpass.getuser()
 
     with patch.dict(cmdmod.__grains__, {"os": "Darwin", "os_family": "Solaris"}):
         stdout = cmdmod._run(cmd, cwd=cwd, runas=runas).get("stdout")