Port tests to async model

[stegut01]

Test reults from mbedgt -VS:

mbedgt: test suite report:
+---------------+---------------+----------------------------------------+--------+--------------------+-------------+
| target        | platform_name | test suite                             | result | elapsed_time (sec) | copy_method |
+---------------+---------------+----------------------------------------+--------+--------------------+-------------+
| frdm-k64f-gcc | K64F          | minar-platform-mbed-test-platform_test | OK     | 9.79               | shell       |
+---------------+---------------+----------------------------------------+--------+--------------------+-------------+
mbedgt: test suite results: 1 OK
mbedgt: test case report:
+---------------+---------------+----------------------------------------+---------------+--------+--------+--------+--------------------+
| target        | platform_name | test suite                             | test case     | passed | failed | result | elapsed_time (sec) |
+---------------+---------------+----------------------------------------+---------------+--------+--------+--------+--------------------+
| frdm-k64f-gcc | K64F          | minar-platform-mbed-test-platform_test | platform_test | 1      | 0      | OK     | 9.79               |
+---------------+---------------+----------------------------------------+---------------+--------+--------+--------+--------------------+
mbedgt: test case results: 1 OK
mbedgt: completed in 10.83 sec

$yt ls
minar-platform-mbed 1.1.3
|_ cmsis-core 1.1.2
| \_ cmsis-core-freescale 1.0.0 yotta_modules\cmsis-core-freescale
|   \_ cmsis-core-k64f 1.0.0 yotta_modules\cmsis-core-k64f
|_ mbed-hal 1.2.2
| |_ mbed-drivers 1.3.0 yotta_modules\mbed-drivers
| | |_ ualloc 1.1.0 yotta_modules\ualloc
| | | \_ dlmalloc 1.0.0 yotta_modules\dlmalloc
| | |_ core-util 1.6.0 yotta_modules\core-util
| | \_ compiler-polyfill 1.2.1 yotta_modules\compiler-polyfill
| \_ mbed-hal-freescale 1.0.0 yotta_modules\mbed-hal-freescale
|   \_ mbed-hal-ksdk-mcu 1.1.0 yotta_modules\mbed-hal-ksdk-mcu
|     |_ uvisor-lib 2.0.0 yotta_modules\uvisor-lib
|     \_ mbed-hal-k64f 1.2.0 yotta_modules\mbed-hal-k64f
|       \_ mbed-hal-frdm-k64f 1.0.2 yotta_modules\mbed-hal-frdm-k64f
|_ minar 1.1.0
| \_ minar-platform 1.0.0 yotta_modules\minar-platform
\_ greentea-client 0.1.8 (test dependency)

[stegut01]

@0xc0170 Please review