devtool: Split tests into multiple classes

This allows better parallelism between the different tests as currently
this block takes the longest time to execute. devtool tests are still
all grouped into the "devtool" module for ease of exection.

This also makes it easier to execute some subset of devtool tests for
testing devtool changes.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>