sage.graphs.is_projective_planar_test: new is_projective_planar() pytests

To ensure that is_projective_planar() still works in the absense of
the cache (now that it is a @cached_method), we add a new pytest file.
For the moment it only tests the Petersen graph, since that is the
only graph with the method initially cached.

Author: orlitzky

src/sage/graphs/is_projective_planar_test.py

@@ -0,0 +1,13 @@
+import pytest
+
+
+def test_petersen_graph_is_projective_planar():
+    r"""
+    Ensure that the Petersen graph is projective planar without
+    using the cache.
+    """
+    from sage.graphs.generators.smallgraphs import PetersenGraph
+    P = PetersenGraph()
+    P.is_projective_planar.clear_cache()
+    assert(not P.is_projective_planar.is_in_cache())
+    assert(P.is_projective_planar())