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/README.md b/test/py/README.md
index 4d9d2b8..2156661 100644
--- a/test/py/README.md
+++ b/test/py/README.md
@@ -310,6 +310,7 @@
 
 - `buildconfig.get(...`
 - `@pytest.mark.buildconfigspec(...`
+- `@pytest.mark.notbuildconfigspec(...`
 
 ### Complete invocation example