Periodically purge in memory-intensive integration tests.

This resolves #393.

test/integration/mallocx.c

@@ -196,6 +196,13 @@ TEST_BEGIN(test_alignment_and_size)
 				}
 			}
 		}
+		/*
+		 * On systems which can't merge extents, this test generates a
+		 * lot of dirty memory very quickly.  Purge between cycles to
+		 * avoid potential OOM on e.g. 32-bit Windows.
+		 */
+		assert_d_eq(mallctl("arena.0.purge", NULL, NULL, NULL, 0), 0,
+		    "Unexpected mallctl error");
 	}
 #undef MAXALIGN
 #undef NITER