unit_tests: env vars are case sensitive

[kolbma]

Next to my pull request #1446 for the SRCDIR variable, I've seen another env var:

const char *s = getenv("unrar_disabled");

https://github.com/kolbma/clamav-devel/blob/86a38352182aef4b63fcbd69fade4ac11b66bea2/unit_tests/check_clamav.c#L541

I don't need this flag, but I'm pretty sure this needs to be at least upper case, too.
But what I see, the check might be correctly on inverted ENABLE_UNRAR variable, because I don't think unrar_disabled would be set anywhere.