Call `greentea_getc()` 8 times, and then `unhexify` once, instead of
calling `receive_byte()`, which inside calls `greentea_getc()` twice,
for every hex digit.
Since Python3 handles encoding differently than Python2,
a change in the way the data is encoded and sent to the target is needed.
1. Change the test data to be sent as hex string
2. Convert the characters to binary bytes.
This is done because the mbed tools translate the encoding differently
(mbed-greentea, and mbed-htrunner)
Don't use the macro name assert. It's technically permitted as long as
<assert.h> is not included, but it's fragile, because it means the
code and any header that it includes must not include <assert.h>.
