bpo-36017: Improve test_grp.test_errors for (big) LDAP directories

There is no guarantee that the group database returned on unix system is
complete. It is typically cut short when LDAP directories are configured
for bigger network setups. This makes it tricky to find a nonexistent
group id by looking at the group database.

This changes the test to pick a nonexistent gid within gid 1-99 which
are typically static assigned and not managed by network directory
systems.

Also pick a group name that is extremely likely to not exist instead of
modifying the name of the first group in the database.

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Author: MatzeB

Lib/test/test_grp.py

@@ -64,7 +64,8 @@ def test_errors(self):
 
         allnames = list(bynames.keys())
         namei = 0
-        fakename = allnames[namei]
+        # Start with a name that very likely does not exist. Typos deliberate.
+        fakename = "should_noot_exxist"
         while fakename in bynames:
             chars = list(fakename)
             for i in range(len(chars)):
@@ -87,12 +88,21 @@ def test_errors(self):
 
         self.assertRaises(KeyError, grp.getgrnam, fakename)
 
-        # Choose a non-existent gid.
-        fakegid = 4127
-        while fakegid in bygids:
-            fakegid = (fakegid * 3) % 0x10000
-
-        self.assertRaises(KeyError, grp.getgrgid, fakegid)
+        # Picking a nonexistent GID is hard, since getgrall() will not
+        # necessarily report all existing groups (typical for LDAP based
+        # directories in big organisations).
+        # Try to find one in the range 1-99 as the lower IDs are typically
+        # statically assigned and more likely to be reported accurately.
+        nonexistent_gid = None
+        candidates = list(range(1, 99))
+        candidates = candidates[90:] + candidates[:90]
+        for i in candidates:
+            if i not in bygids:
+                nonexistent_gid = i
+                break
+
+        if nonexistent_gid is not None:
+            self.assertRaises(KeyError, grp.getgrgid, nonexistent_gid)
 
     def test_noninteger_gid(self):
         entries = grp.getgrall()