[dvs] Re-add runcmd_async command (sonic-net#1508)

Signed-off-by: Danny Allen <[email protected]>

Author: daall

tests/conftest.py

@@ -199,6 +199,10 @@ def runcmd(self, cmd: str) -> int:
 
         return 0
 
+    # used in buildimage tests, do not delete
+    def runcmd_async(self, cmd: str) -> subprocess.Popen:
+        return subprocess.Popen(f"ip netns exec {self.nsname} {cmd}", shell=True)
+
     def runcmd_output(self, cmd: str) -> str:
         return subprocess.check_output(f"ip netns exec {self.nsname} {cmd}", shell=True).decode("utf-8")