tests/timer_api: Use correct 'abs' macro with correct type parameter

Subtracting with a uint64_t operand yields a uint64_t result, for which
the absolute value is not terribly interesting. Cast the operand to
int64_t.

Use llabs instead of abs as abs takes an int parameter and not an
int64_t. This appears to work even with the minimal C library.

Signed-off-by: Keith Packard <keithp@keithp.com>