pytest: Add a `compat()` fixture allowing checks for specific flags

Since we change the interface for `pay` and `paystatus` we need a way to
determine which one to expect. This just parses the COMPAT_Vxyz flags and
allows us to check if, based on the configuration, we are in compat mode for
that set of changes or not.

tests/fixtures.py

@@ -1,8 +1,11 @@
 from utils import DEVELOPER, TEST_NETWORK  # noqa: F401,F403
 from pyln.testing.fixtures import directory, test_base_dir, test_name, chainparams, node_factory, bitcoind, teardown_checks, db_provider, executor, setup_logging  # noqa: F401,F403
 from pyln.testing import utils
+from utils import COMPAT
 
+import os
 import pytest
+import re
 
 
 @pytest.fixture
@@ -17,3 +20,28 @@ class LightningNode(utils.LightningNode):
         # Yes, we really want to test the local development version, not
         # something in out path.
         self.daemon.executable = 'lightningd/lightningd'
+
+
+class CompatLevel(object):
+    """An object that encapsulates the compat-level of our build.
+    """
+    def __init__(self):
+        makefile = os.path.join(os.path.dirname(__file__), "..", "Makefile")
+        lines = [l for l in open(makefile, 'r') if l.startswith('COMPAT_CFLAGS')]
+        assert(len(lines) == 1)
+        line = lines[0]
+        flags = re.findall(r'COMPAT_V([0-9]+)=1', line)
+        self.compat_flags = flags
+
+    def __call__(self, version):
+        return COMPAT and version in self.compat_flags
+
+
+@pytest.fixture
+def compat():
+    return CompatLevel()
+
+
+def is_compat(version):
+    compat = CompatLevel()
+    return compat(version)