tests: native_posix: use llabs with int64_t

subpress warnings from llvm:

warning: absolute value function 'abs' given an argument of type
'int64_t' (aka 'long long') but has parameter of type 'int' which may
cause truncation of value [-Wabsolute-value]

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
1 file changed