selftests: drivers: add scaffolding for Netlink tests in Python

Add drivers/net as a target for mixed-use tests.
The setup is expected to work similarly to the forwarding tests.
Since we only need one interface (unlike forwarding tests)
read the target device name from NETIF. If not present we'll
try to run the test against netdevsim.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/drivers/net/lib/py/env.py

@@ -0,0 +1,52 @@
+# SPDX-License-Identifier: GPL-2.0
+
+import os
+import shlex
+from pathlib import Path
+from lib.py import ip
+from lib.py import NetdevSimDev
+
+class NetDrvEnv:
+    def __init__(self, src_path):
+        self._ns = None
+
+        self.env = os.environ.copy()
+        self._load_env_file(src_path)
+
+        if 'NETIF' in self.env:
+            self.dev = ip("link show dev " + self.env['NETIF'], json=True)[0]
+        else:
+            self._ns = NetdevSimDev()
+            self.dev = self._ns.nsims[0].dev
+        self.ifindex = self.dev['ifindex']
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, ex_type, ex_value, ex_tb):
+        """
+        __exit__ gets called at the end of a "with" block.
+        """
+        self.__del__()
+
+    def __del__(self):
+        if self._ns:
+            self._ns.remove()
+            self._ns = None
+
+    def _load_env_file(self, src_path):
+        src_dir = Path(src_path).parent.resolve()
+        if not (src_dir / "net.config").exists():
+            return
+
+        lexer = shlex.shlex(open((src_dir / "net.config").as_posix(), 'r').read())
+        k = None
+        for token in lexer:
+            if k is None:
+                k = token
+                self.env[k] = ""
+            elif token == "=":
+                pass
+            else:
+                self.env[k] = token
+                k = None