kernel: k_cpu_idle/atomic_idle() not tested for tick-less kernel

[ioannisg]

Describe the bug
The kernel CPU idle-ing APIs are not test-covered when kernel runs in tick-less mode.

To Reproduce
From tests/kernel/context:

#if defined(CONFIG_TICKLESS_KERNEL)
static void test_kernel_cpu_idle(void)
{
	ztest_test_skip();
}
static void test_kernel_cpu_idle_atomic(void)
{
	ztest_test_skip();
}

Impact
The API is used in the idle thread so it should be tested for tick-less kernels.

[ioannisg]

CC @andyross @andrewboie