infamy: Fix default transport selection

Using `sys.argv[0]` does not quite achieve the effect we want, namely
that any (test-case, $PYTHONHASHSEED) tuple should always default to
the same transport.

For example, the following two examples should always use the same
transport:

    $ make PYTHONHASHSEED=1337 test-sh
    infamy0:test$ ./case/ietf_system/hostname/test.py

    $ make PYTHONHASHSEED=1337 test-sh
    infamy0:test$ cd ./case/ietf_system/hostname
    infamy0:hostname$ ./test.py

But, since their argv[0]'s are different, they don't.

Therefore use the _last two components_ of argv[0]'s full path
instead. This should:

- Spread the allocation over an entire suite run, since the
  penultimate component is usually the test name.

- Keep the allocation stable when test code is modified (which is
  common during debugging of failing tests)

- Avoid instabilities stemming from the local storage location (i.e.,
  home directory names etc.)

test/infamy/env.py

@@ -18,6 +18,20 @@ ENV = NullEnv()
 
 
 class ArgumentParser(argparse.ArgumentParser):
+    def DefaultTransport():
+        """Pick pseudo-random transport
+
+        If the user does not specify a particular transport, make sure
+        that any (test, $PYTHONHASHSEED) tuple will always map to the
+        same transport.
+
+        """
+        name = "/".join(os.path.realpath(sys.argv[0]).split(os.sep)[-2:])
+        seed = os.environ.get('PYTHONHASHSEED', 0)
+        random.seed(f"{name}-{seed}")
+
+        return random.choice(["netconf", "restconf"])
+
     def __init__(self, top):
         super().__init__()
 
@@ -25,7 +39,7 @@ class ArgumentParser(argparse.ArgumentParser):
         self.add_argument("-l", "--logical-topology", dest="ltop", default=top)
         self.add_argument("-p", "--package", default=None)
         self.add_argument("-y", "--yangdir", default=None)
-        self.add_argument("-t", "--transport", default=None)
+        self.add_argument("-t", "--transport", default=ArgumentParser.DefaultTransport())
         self.add_argument("ptop", nargs=1, metavar="topology")
 
 
@@ -84,15 +98,12 @@ class Env(object):
         else:
             mapping = None
 
-        # Test protocol always highest prio, followed by command line,
-        # then environment (detected from defconfig), lastly random.
+        # Precedence:
+        # 1. Caller specifies `protocol`
+        # 2. User specifies `-t` when executing test
+        # 3. One is pseudo-randomly picked based on $PYTHONHASHSEED
         if protocol is None:
-            if self.args.transport is not None:
-                protocol = self.args.transport
-            else:
-                hseed = os.environ.get('PYTHONHASHSEED', 0)
-                random.seed(f"{sys.argv[0]}-{hseed}")
-                protocol = random.choice(["netconf", "restconf"])
+            protocol = self.args.transport
 
         if password is None:
             password = self.get_password(node)