Fix graphviz check in pass manager visualization

This commit fixes the graphviz check in the pass manager visualization
added recently in Qiskit#2445. The check was added to determine if the dot
command was in the default PATH by trying to subprocess out and run
'dot -V' however the way in which subprocess was called results in both
the dot version string and a blank line being printed if graphviz is
actually installed. This happens on import time which we definitely
don't want to do (and breaks test discovery in stestr).

qiskit/visualization/pass_manager_visualization.py

@@ -23,9 +23,14 @@
 
 try:
     import subprocess
-    print(subprocess.check_output(['dot', '-V']))
-    HAS_GRAPHVIZ = True
-except FileNotFoundError:
+    proc = subprocess.Popen(['dot', '-V'], stdout=subprocess.PIPE,
+                            stderr=subprocess.PIPE)
+    proc.communicate()
+    if proc.returncode != 0:
+        HAS_GRAPHVIZ = False
+    else:
+        HAS_GRAPHVIZ = True
+except Exception:
     # this is raised when the dot command cannot be found, which means GraphViz
     # isn't installed
     HAS_GRAPHVIZ = False