WIP: enable live logging for non-parallel test runs

This provides incremental output when test is running _without xdist_,
just like the old runner did.

With xdist the live output is not available, I believe because of
pytest-dev/pytest-xdist#402
pytest-dev/pytest-xdist#883 might help with
that, but I'm not going to hold my breath until it is available on
distros we use.

bin/tests/system/conftest.py

@@ -364,28 +364,25 @@ def _run_script(  # pylint: disable=too-many-arguments
             raise FileNotFoundError(f"script {script} not found in {cwd}")
         logger.debug("running script: %s %s %s", interpreter, script, " ".join(args))
         logger.debug("  workdir: %s", cwd)
-        stdout = b""
         returncode = 1
-        try:
-            proc = subprocess.run(
-                [interpreter, script] + args,
-                env=env,
-                check=True,
-                stdout=subprocess.PIPE,
-                stderr=subprocess.STDOUT,
-            )
-        except subprocess.CalledProcessError as exc:
-            stdout = exc.stdout
-            returncode = exc.returncode
-            raise exc
-        else:
-            stdout = proc.stdout
-            returncode = proc.returncode
-        finally:
-            if stdout:
-                for line in stdout.decode().splitlines():
-                    logger.debug("    %s", line)
-            logger.debug("  exited with %d", returncode)
+
+        cmd = [interpreter, script] + args
+        proc = subprocess.Popen(
+            cmd,
+            env=env,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            bufsize=1,
+            universal_newlines=True
+        )
+        if proc.stdout:
+            for line in proc.stdout:
+                logger.info("    %s", line.rstrip('\n'))
+        proc.communicate()
+        returncode = proc.returncode
+        if returncode:
+            raise subprocess.CalledProcessError(returncode, cmd)
+        logger.debug("  exited with %d", returncode)
         return proc
 
     @pytest.fixture(scope="module")