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The isctest.run.Dig() is superseded with the isctest.run.EnvCmd().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().

bin/tests/system/isctest/run.py

@@ -81,6 +81,19 @@ def cmd(
         return CmdResult(exc)
 
 
+class EnvCmd:
+    """Helper for executing binaries from env with optional base parameters."""
+
+    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 _run_script(
     interpreter: str,
     script: str,
@@ -116,17 +129,6 @@ def _run_script(
         isctest.log.debug("  exited with %d", returncode)
 
 
-class Dig:
-    def __init__(self, base_params: str = ""):
-        self.base_params = base_params
-
-    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 shell(script: str, args: Optional[List[str]] = None) -> None:
     """Run a given script with system's shell interpreter."""
     _run_script(os.environ["SHELL"], script, args)

bin/tests/system/keepalive/tests_keepalive.py

@@ -28,7 +28,7 @@ def test_dig_tcp_keepalive_handling(named_port, ns2):
                     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