Skip extmu ML-DSA variants in generic signature tests (#152)

liboqs main now enables the external-mu ML-DSA variants (ML-DSA-44-extmu,
ML-DSA-65-extmu, ML-DSA-87-extmu). These cannot be signed via the standard
OQS_SIG_sign path, so the generic correctness tests in test_sig.py, which
iterate over all enabled mechanisms and call plain sign(), fail with
"RuntimeError: Can not sign message".

Exclude the extmu variants from the generic sign/verify tests to unblock CI.
Proper support (a dedicated external-mu signing path plus tests) is tracked
in #151.

Signed-off-by: Douglas Stebila <dstebila@uwaterloo.ca>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dstebila

tests/test_sig.py

@@ -4,8 +4,13 @@
 import oqs
 from oqs.oqs import Signature, native
 
-# Sigs for which unit testing is disabled
-disabled_sig_patterns = []
+# Sigs for which unit testing is disabled.
+#
+# The ML-DSA "external mu" (extmu) variants require an externally computed mu
+# and cannot be signed via the standard sign() API, so exclude them from the
+# generic sign/verify correctness tests. See
+# https://github.com/open-quantum-safe/liboqs-python/issues/151.
+disabled_sig_patterns = ["extmu"]
 
 if platform.system() == "Windows":
     disabled_sig_patterns = [""]