tests: timer: Unstable tests using timer at nrf platforms

[PerMac]

Describe the bug
Once in a few runs tests
tests/kernel/timer/timer_api/: test_timeout_abs (sometimes fails on nrf9160dk_nrf9160, often on nrf5340dk_nrf5340_cpuappns )
tests/posix/common/ : test_posix_realtime (sometimes at nrf9160, often on nrf5340dk_nrf5340_cpuappns)
are failing, respectively:

START - test_timeout_abs
Assertion failed at WEST_TOPDIR/zephyr/tests/kernel/timer/timer_api/src/main.c:777: test_timeout_abs: (t0 + rem_ticks == exp_ticks is false)
Wrong remaining: now 135407 rem 327544594 expires 327680000 (1)
FAIL - test_timeout_abs in 0.20 seconds

START - test_posix_realtime
POSIX clock set APIs
Assertion failed at WEST_TOPDIR/zephyr/tests/posix/common/src/clock.c:75: test_posix_realtime: (rts.tv_nsec not equal to mts.tv_nsec)
Nanoseconds not equal
FAIL - test_posix_realtime

I created a combined issue as there might be the same root cause for these. Sorry for such imprecise language (often, sometimes). I haven't counted the success/fail rate, just run it several times in a row, also saw these test sometimes failing in our on-target CI. It is possible that the tests are also unstable on other (not mentioned here) nrf platforms, but I haven't caught it.

Environment:

• OS: Ubuntu 18.04
• Toolchain zephyr-sdk 0.12.2
• Commit SHA or Version used v2.6.0-rc1-244-gaa78a6473b16

Additional context
Add any other context about the problem here.