Various tests for semantics

Empty semantics (invalid):

annotation (invalid): annotation

annotation-xml (invalid): annotation-xml

presentation MathML (no annotations): presentation MathML

content MathML (no annotations): content MathML

presentation MathML ; annotation: presentation MathML annotation

presentation MathML ; annotation-xml: presentation MathML annotation-xml

content MathML ; annotation: content MathML annotation

content MathML ; annotation-xml: content MathML annotation-xml