[9.18] new: test: Add nsX fixtures to pytest

The nsX are utility fixtures which can be used instead of the servers
fixture, which requires longer syntax(i.e. servers["nsX"]).

---

This MR is basically backporting pytest utility fixtures which were introduced to newer branches in !10717. This is a minimal change which only aims to facilitate easier test backports to ~"v9.18" in the future, without changing any of the existing tests.

Merge branch 'nicki/pytest-nsX-fixtures-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11015

bin/tests/system/conftest.py

@@ -717,7 +717,7 @@ def system_test(
         request.node.stash[FIXTURE_OK] = True
 
 
-@pytest.fixture
+@pytest.fixture(scope="module")
 def servers(ports, system_test_dir):
     instances = {}
     for entry in system_test_dir.rglob("*"):
@@ -733,3 +733,53 @@ def servers(ports, system_test_dir):
             except ValueError:
                 continue
     return instances
+
+
+@pytest.fixture(scope="module")
+def ns1(servers):
+    return servers["ns1"]
+
+
+@pytest.fixture(scope="module")
+def ns2(servers):
+    return servers["ns2"]
+
+
+@pytest.fixture(scope="module")
+def ns3(servers):
+    return servers["ns3"]
+
+
+@pytest.fixture(scope="module")
+def ns4(servers):
+    return servers["ns4"]
+
+
+@pytest.fixture(scope="module")
+def ns5(servers):
+    return servers["ns5"]
+
+
+@pytest.fixture(scope="module")
+def ns6(servers):
+    return servers["ns6"]
+
+
+@pytest.fixture(scope="module")
+def ns7(servers):
+    return servers["ns7"]
+
+
+@pytest.fixture(scope="module")
+def ns8(servers):
+    return servers["ns8"]
+
+
+@pytest.fixture(scope="module")
+def ns9(servers):
+    return servers["ns9"]
+
+
+@pytest.fixture(scope="module")
+def ns10(servers):
+    return servers["ns10"]