Add generic isctest.run.EnvCmd helper to pytest

A generic helper that calls the environment-specified binaries in a
developer-friendly manner, i.e. passing arguments as strings rather than
having to split them first.

The isctest.run.cmd() remains as the basis which provides a clean and
robust interface, while the isctest.run.EnvCmd() can be used as a
convenient wrapper for tests, or when there are some shared default
parameters.

The isctest.run.Dig() is superseded with the isctest.run.EnvCmd(). In
the future, we might revisit adding Dig() or command-specific helpers
again, but it probably only makes sense if they offer command-aware
attributes / methods, rather than just being shortcuts to
isctest.run.EnvCmd().

(cherry picked from commit ff613a72d7)

bin/tests/system/isctest/run.py

@@ -83,15 +83,17 @@ def cmd(
         return CmdResult(exc)
 
 
-class Dig:
-    def __init__(self, base_params: str = ""):
-        self.base_params = base_params
+class EnvCmd:
+    """Helper for executing binaries from env with optional base parameters."""
 
-    def __call__(self, params: str) -> CmdResult:
-        """Run the dig command with the given parameters."""
-        return cmd(
-            [os.environ.get("DIG")] + f"{self.base_params} {params}".split(),
-        )
+    def __init__(self, name: str, base_params: str = ""):
+        self.bin_path = os.environ[name]
+        self.base_params = base_params.split()
+
+    def __call__(self, params: str, **kwargs) -> CmdResult:
+        """Call the command. Keyword arguments from isctest.run.cmd() are supported."""
+        args = self.base_params + params.split()
+        return cmd([self.bin_path] + args, **kwargs)
 
 
 def retry_with_timeout(func, timeout, delay=1, msg=None):

bin/tests/system/keepalive/tests_keepalive.py

@@ -28,7 +28,7 @@ def test_dig_tcp_keepalive_handling(named_port, servers):
                     options_received = line.split()[0]
         return int(options_received)
 
-    dig = isctest.run.Dig(f"-p {str(named_port)}")
+    dig = isctest.run.EnvCmd("DIG", f"-p {str(named_port)}")
 
     isctest.log.info("check that dig handles TCP keepalive in query")
     assert "; TCP KEEPALIVE" in dig("+qr +keepalive foo.example. @10.53.0.2").out