APIv2 (python) tests: fix flake

Python tests were flaking because they behave differently
when $DEBUG is set. It looks like something in CI sets
that envariable.

Solution: do not use $DEBUG as a debug trigger, use a
properly-named custom variable that is unlikely to be
set accidentally.

Also: get rid of AssertTrue(), which gives no visibility
into what happened. Write in proper form that can emit
useful diagnostics on failure.

Fixes: #10948

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/python/docker/__init__.py

@@ -27,7 +27,7 @@ class Podman(object):
         # No support for tmpfs (/tmp) or extfs (/var/tmp)
         # self.cmd.append("--storage-driver=overlay")
 
-        if os.getenv("DEBUG"):
+        if os.getenv("PODMAN_PYTHON_TEST_DEBUG"):
             self.cmd.append("--log-level=debug")
             self.cmd.append("--syslog=true")