test: env: Add support for querying global attributes

This will let other Infamy submodules tune their behavior based on the
particular test system they are running on.

Attributes are defined by creating graph-global attributes with an
"ix_" prefix.

test/infamy/env.py

@@ -7,6 +7,12 @@ import sys
 
 from . import neigh, netconf, tap, topology
 
+class NullEnv:
+    def attr(self, _, default=None):
+        return default
+
+ENV = NullEnv()
+
 class ArgumentParser(argparse.ArgumentParser):
     def __init__(self, ltop):
         super().__init__()
@@ -37,6 +43,21 @@ class Env(object):
 
             print(repr(self.ltop))
 
+        global ENV
+        ENV = self
+
+    def attr(self, name, default=None):
+        val = self.ptop.get_attr("ix_" + name)
+        if val == None:
+            return default
+
+        try:
+            return int(val, 0)
+        except ValueError:
+            pass
+
+        return val
+
     def attach(self, node, port, factory_default=True):
         if self.ltop:
             mapping = self.ltop.mapping[node]