The `clock()` function reads the current second and microsecond value of
the system clock, optionally using the monotonic clock instead of the
default realtime one.

-- Testcase --
{%
	let t1 = clock();
	let t3 = clock(true);

	sleep(250);

	let t2 = clock();
	let t4 = clock(true);

	let delta1 = (t2[0] - t1[0]) * 1000000000 + (t2[1] - t1[1]);
	let delta2 = (t4[0] - t3[0]) * 1000000000 + (t4[1] - t3[1]);

	assert(delta1 >= 0, "Realtime clock went backwards!");
	assert(delta2 >= 0, "Monotonic clock went backwards!");

	printf("%.J\n", [
		(delta1 >= 240000000 && delta1 <= 260000000) ? true : "unexpected delta: " + delta1,
		(delta2 >= 240000000 && delta2 <= 260000000) ? true : "unexpected delta: " + delta2
	]);
%}
-- End --

-- Expect stdout --
[
	true,
	true
]
-- End --