Add async tests

We got some async test in the mbed-os, that shall be added/or rewrite or write a new one.

To be added:
- SPI
- I2C
- serial

Tests references:
- https://github.com/ARMmbed/mbed-os/blob/master/targets/TARGET_Silicon_Labs/TARGET_EFM32/TESTS/spi/basic-spi/main.cpp
- https://github.com/ARMmbed/mbed-os/tree/master/features/unsupported/tests/utest (async tests)

Start with the referenced SPI example at targets/TARGET_Silicon_Labs/TARGET_EFM32/TESTS/spi/basic-spi/main.cpp and review the async tests under features/unsupported/tests/utest. Determine how SPI, I2C, and serial coverage should be added or rewritten, then verify that async tests exist for all three interfaces.

Written by the indexing model from the issue text.