test/py: pytest.mark.notbuildconfigspec()

We already can let a Python test depend on a build option being set via
@pytest.mark.buildconfigspec(). It may be necessary to let a test depend on
a build option *not* being set. So let's introduce

    @pytest.mark.notbuildconfigspec

for this purpose.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/test/py/conftest.py b/test/py/conftest.py
index e40cbf0..00d8ef8 100644
--- a/test/py/conftest.py
+++ b/test/py/conftest.py
@@ -460,11 +460,15 @@
     """
 
     mark = item.get_marker('buildconfigspec')
-    if not mark:
-        return
-    for option in mark.args:
-        if not ubconfig.buildconfig.get('config_' + option.lower(), None):
-            pytest.skip('.config feature "%s" not enabled' % option.lower())
+    if mark:
+        for option in mark.args:
+            if not ubconfig.buildconfig.get('config_' + option.lower(), None):
+                pytest.skip('.config feature "%s" not enabled' % option.lower())
+    notmark = item.get_marker('notbuildconfigspec')
+    if notmark:
+        for option in notmark.args:
+            if ubconfig.buildconfig.get('config_' + option.lower(), None):
+                pytest.skip('.config feature "%s" enabled' % option.lower())
 
 def tool_is_in_path(tool):
     for path in os.environ["PATH"].split(os.pathsep):