Fix bwrap leak test

Our checking of processes during bwrap leak testing is racy. We handle
these races by ignoring a couple exceptions that may happen due to
things moving while we iterate over them. However if this happens the
first time through the loop sleep_proc won't ever be set then we check
its value which is itself an exception.

Address this by checking the value of sleep_proc within the try block so
that we know no exceptions due to races occured and the value is set.

Change-Id: Iea6c7539dceaa3939dcdef5abeccff6481f837f8

tests/unit/test_bubblewrap.py

@@ -74,10 +74,10 @@ class TestBubblewrap(testtools.TestCase):
                 sleep_proc = [pid for pid in os.listdir("/proc") if
                               os.path.isfile("/proc/%s/cmdline" % pid) and
                               open("/proc/%s/cmdline" % pid).read() == cmdline]
+                if not sleep_proc:
+                    break
             except FileNotFoundError:
                 pass
             except ProcessLookupError:
                 pass
-            if not sleep_proc:
-                break
             time.sleep(1)