Target-specific test gets compiled for incorrect targets

[bridadan]

A target-specific test was added recently: https://github.com/mbedmicro/mbed/blob/da2a4ccbaeb8218f8f5e97e26b4ebd916be46ccd/hal/targets/hal/TARGET_Silicon_Labs/TARGET_EFM32/TESTS/spi/basic-spi/main.cpp. This came as a part of da2a4cc.

The test discovery logic does not currently take into account TARGET_, TOOLCHAIN_, nor FEATURE_ prefixes when scanning for tests. This is a deficiency as this should not be a special case. It should follow the same behavior as the build system.

cc @akselsm

[ciarmcom]

ARM Internal Ref: IOTMORF-141

[bridadan]

Fixed by #2244