testsuite: Bump margin of T4029 to 15%

This test has been fluctuating wildly recently. Moreover, it's not even clear to
me that this is a particularly useful thing to be testing.

testsuite/tests/perf/space_leaks/all.T

@@ -69,7 +69,7 @@ test('T4029',
             # 2017-03-03: 65 (amd64/Linux)           Share Typeable KindReps or more
             #                                        lazy interface file reading
       stats_num_field('max_bytes_used',
-          [(wordsize(64), 18208944, 5)]),
+          [(wordsize(64), 18208944, 15)]),
             # 2016-02-26: 24071720 (amd64/Linux)     INITIAL
             # 2016-04-21: 25542832 (amd64/Linux)
             # 2016-05-23: 25247216 (amd64/Linux)     Use -G1
@@ -86,6 +86,7 @@ test('T4029',
             #                                        lazy interface file reading
             # 2017-03-07: 20476360 (amd64/Linux)     It's not entirely clear
             # 2017-03-14: 18208944 (amd64/Darwin)    Again, not clear
+            # 2017-03-15: bumped margin to 15% due to instability
       extra_hc_opts('+RTS -G1 -RTS' ),
       ],
      ghci_script,