Add parenthesis around the BIT_TEST() macro.

This commit adds encapsulating parenthesis around the BIT_TEST() macro
in bits.h. This allows caller's to safely treat the macro as an atomic
operation, rather than a series of expressions with competing
precedence.

Specifically, this fixes a bug on line 204 of leornian.c where !! is
used to negate the value returned by BIT_TEST().