Periodically purge in memory-intensive integration tests.

This resolves #393.

test/integration/mallocx.c

@@ -139,6 +139,13 @@ TEST_BEGIN(test_basic)
 		rsz = sallocx(p, 0);
 		assert_zu_eq(nsz, rsz, "nallocx()/sallocx() rsize mismatch");
 		dallocx(p, 0);
+		/*
+		 * 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 MAXSZ
 }