Do not set MALLOC_PERTURB_ for benchmarks

Setting MALLOC_PERTURB_ to a non-zero value is fine for regular test
cases. It helps catching bugs, but also comes with some runtime
overhead.

This overhead is noticeable for benchmarks when compared to running them
directly instead of through Meason.

Therefore, MALLOC_PERTURB_ is not touched for benchmarks.

closes #3034

mesonbuild/mtest.py

@@ -228,10 +228,10 @@ class TestHarness:
 
             # If MALLOC_PERTURB_ is not set, or if it is set to an empty value,
             # (i.e., the test or the environment don't explicitly set it), set
-            # it ourselves. We do this unconditionally because it is extremely
-            # useful to have in tests.
+            # it ourselves. We do this unconditionally for regular tests
+            # because it is extremely useful to have.
             # Setting MALLOC_PERTURB_="0" will completely disable this feature.
-            if 'MALLOC_PERTURB_' not in child_env or not child_env['MALLOC_PERTURB_']:
+            if ('MALLOC_PERTURB_' not in child_env or not child_env['MALLOC_PERTURB_']) and not self.options.benchmark:
                 child_env['MALLOC_PERTURB_'] = str(random.randint(1, 255))
 
             setsid = None