Added MBED_HOSTTEST_ASSERT
Added MBED_HOSTTEST_ASSERT

This is a macro that I've used quite a bit in my tests. The main advantage over
the other MBED_HOSTTEST_ macros is that it'll display the condition that failed
and the location of the failure, similar to MBED_ASSERT.

sg-/mbed-drivers,bremoran/mbed-drivers,bremoran/mbed-drivers,0xc0170/mbed-drivers,iriark01/mbed-drivers,sg-/mbed-drivers,0xc0170/mbed-drivers,iriark01/mbed-drivers