tests: context: move idle test to the end

On some devices, when k_cpu_idle() was called we were getting
interrupts that were not the timer interrupt. On bbc_micro
a power clock control driver interrupt was happening instead
and k_cpu_idle() was returning without the system tick advancing,
failing the test.

The clock control interrupts seem to only happen early in device
boot; moving the idle test much later lets the test pass on this
board (and likely all other NRF5 based boards).

Issue: ZEP-2257
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>