fix: two issues with the N3 serializer

This patch fixes two issues with the N3 serializer:
- The N3 serializer incorrectly considered a subject as already
  serialized if it has been serialized inside a quoted graph.
- The N3 serializer does not consider that the predicate of
  a triple can also be a graph.

Other changes included in this patch:
- Changed `test.testutils.GraphHelper` to support nested/quoted graphs.
- Moved the tests from `test/test_n3_formula.py` into
  `test/test_serializers/test_serializer_n3.py`.
- Include positive syntax tests from the N3 test suite that is smaller
  than 1024KB and that is not using new N3 syntax into round trip tests.
  This is mainly to check that there is no regressions after the changes
  made.

Fixes:
- #1807
- #1701

rdflib/plugins/serializers/n3.py

@@ -23,36 +23,6 @@ def reset(self):
         super(N3Serializer, self).reset()
         self._stores = {}
 
-    def subjectDone(self, subject):
-        super(N3Serializer, self).subjectDone(subject)
-        if self.parent:
-            self.parent.subjectDone(subject)
-
-    def isDone(self, subject):
-        return super(N3Serializer, self).isDone(subject) and (
-            not self.parent or self.parent.isDone(subject)
-        )
-
-    def startDocument(self):
-        super(N3Serializer, self).startDocument()
-        # if not isinstance(self.store, N3Store):
-        #    return
-        #
-        # all_list = [self.label(var) for var in
-        #        self.store.get_universals(recurse=False)]
-        # all_list.sort()
-        # some_list = [self.label(var) for var in
-        #        self.store.get_existentials(recurse=False)]
-        # some_list.sort()
-        #
-        # for var in all_list:
-        #    self.write('\n'+self.indent()+'@forAll %s. '%var)
-        # for var in some_list:
-        #    self.write('\n'+self.indent()+'@forSome %s. '%var)
-        #
-        # if (len(all_list) + len(some_list)) > 0:
-        #    self.write('\n')
-
     def endDocument(self):
         if not self.parent:
             super(N3Serializer, self).endDocument()
@@ -68,6 +38,9 @@ def preprocessTriple(self, triple):
         if isinstance(triple[0], Graph):
             for t in triple[0]:
                 self.preprocessTriple(t)
+        if isinstance(triple[1], Graph):
+            for t in triple[1]:
+                self.preprocessTriple(t)
         if isinstance(triple[2], Graph):
             for t in triple[2]:
                 self.preprocessTriple(t)