DRuntime: Use atomic ops in host test

As per TDPL and [Issue 3672](https://d.puremagic.com/issues/show_bug.cgi?id=3672), read-modify-write operations should not be allowed for shared variables. Even though it hasn't yet been fixed in the compiler, such usage is erroneous. This is a fix for one of DRuntime tests that replaces plain operations with atomic ones.