tests-mbed_drivers-timer: change delay method

[maciejbocianski]

Description

The test sometimes fails on NRF51_DK (test case: "Test: Timer - time measurement 1 ms.") in morph tests.

Fail log

[1533848813.22][CONN][RXD] >>> Running case #11: 'Test: Timer - time measurement 1 ms.'...
[1533848813.32][CONN][RXD] :730::FAIL: Expected 0.001000 Was 0.001633 
[1533848813.32][CONN][INF] found KV pair in stream: {{__testcase_start;Test: Timer - time measurement 1 ms.}}, queued... 
[1533848813.42][CONN][INF] found KV pair in stream: {{__testcase_finish;Test: Timer - time measurement 1 ms.;0;1}}, queued... 
[1533848813.52][CONN][RXD] >>> 'Test: Timer - time measurement 1 ms.': 0 passed, 1 failed with reason 'Assertion Failed'

The test verifies if Timer class correctly counts elapsed time. Sometimes we got measured ~1600us for delay 1000 us (delta 550 us). The delay is performed using wait_us() function which for delays greater than or equal to 1 ms (our case) calls Thread::wait((uint32_t)ms);. This causes rescheduling and potentially can put board into sleep (deep sleep mode is disabled by wait_us()). For our test purposes we don't need escheduling/sleep since this actions takes extra time and have influence on the time measurement accuracy. The solution is to implement function for delay which is based on busy loop and uses us ticker. It has been verified that this solves the problem. With this fix when test case is repeated 1000 times we got usually measured time equal to ~1080 us, and sometimes ~1300us (checked that this is caused by systick interrupt handling). Since this is test for drivers layer and the results are acceptable I decided to not disabling systick in the test).

Related PR's: #7471

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Feature
[ ] Breaking change

[fkjagodzinski]

👍 for the detailed description.

[jamesbeyond]

Look good

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2783
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7752/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2414
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7752/

[mbed-ci]

Test : SUCCESS

Build number : 2520
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7752/2520