MB-66783: Don't fail MemTrackingBucketTest if arena-debug already set

The test class forces CB_ARENA_MALLOC_VERIFY_DEALLOC_CLIENT, just
continue test execution if that is already enabled in the environment.

Change-Id: I1ef6f7d8a15c68be3221dd52a90f23bb08599f69
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/245423
Reviewed-by: Chirag Agarwal <chirag.agarwal@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Mohammad Zaeem <mohammad.zaeem@couchbase.com>

Author: paolococchi

tests/testapp/testapp_bucket.cc

@@ -395,8 +395,9 @@ class MemTrackingBucketTest : public BucketTest {
         // Note: Important to set the env BEFORE starting memcached,
         // env vars wouldn't be passed to the memcached process otherwise
         // (unless testapp is run in -e (embedded) mode)
-        ASSERT_FALSE(getenv("CB_ARENA_MALLOC_VERIFY_DEALLOC_CLIENT"));
-        ASSERT_EQ(0, setenv("CB_ARENA_MALLOC_VERIFY_DEALLOC_CLIENT", "1", 0));
+        if (!getenv("CB_ARENA_MALLOC_VERIFY_DEALLOC_CLIENT")) {
+            ASSERT_EQ(0, setenv("CB_ARENA_MALLOC_VERIFY_DEALLOC_CLIENT", "1", 0));
+        }
         BucketTest::SetUpTestCase();
     }