Patch failing syntactic tests in special cases

[SolunarNexus]

Modify behaviour in exceptional cases where our validation is "better" or "more strict" and throws additional issues (besides expected ones).

See, e.g. syntactic/10311/10311-fail-01-29-10313-sev2-l3v2.xml where validation correctly records issue 10311 (syntax violation of attribute value), but 10313 as well (identifier not found). That is logically OK, but the syntactic test suite expects only issue 10311 to be recorded.