Widen acceptance window of `MultiLayerModules` (#19293) [skip ci]

As #19293 realises, this one keeps on flip flopping by 2.5%
depending on how many modules there are within the GHC package.

We should revert this once we figured out how to fix what's going on.

testsuite/tests/perf/compiler/all.T

@@ -260,8 +260,11 @@ test('T13379',
      compile,
      [''])
 
+# MultiLayerModules flip flops by 2.5%, depending on the
+# number of modules within GHC (#19293). Just widen the
+# acceptance window until we figured out how to fix it.
 test('MultiLayerModules',
-     [ collect_compiler_stats('bytes allocated',2),
+     [ collect_compiler_stats('bytes allocated',3),
        pre_cmd('./genMultiLayerModules'),
        extra_files(['genMultiLayerModules']),
        compile_timeout_multiplier(5)