Ensure assertions in Graph.apply are asserted.

The Graph.apply test in GraphSuite had some assertions in a closure in
a graph transformation. This caused two problems:

1. because assert() was called, test classes were reachable from the
closures, which made them not serializable, and

2. (more importantly) these assertions never actually executed, since
they occurred within a lazy map()

This commit simply changes the Graph.apply test to collects the graph
triplets so it can assert about each triplet from a map method.

graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala

@@ -62,7 +62,7 @@ class GraphSuite extends FunSuite with LocalSparkContext {
       assert( graph.edges.count() === rawEdges.size )
       // Vertices not explicitly provided but referenced by edges should be created automatically
       assert( graph.vertices.count() === 100)
-      graph.triplets.map { et =>
+      graph.triplets.collect.map { et =>
         assert((et.srcId < 10 && et.srcAttr) || (et.srcId >= 10 && !et.srcAttr))
         assert((et.dstId < 10 && et.dstAttr) || (et.dstId >= 10 && !et.dstAttr))
       }