tools: tests: port tests to shunit2

tools: tests: port tests to shunit2

BATS has been confirmed to run much more slowly that shunit2. This is
most likely caused by the way BATS evaluates each test file n+1 times
where n is the number of tests[1].

Port tests to using shunit2 which executes as a regular shell script
which, in addition to higher speed, allows for easier debugging as
standard shell flags like -x now work.

[1] https://bats-core.readthedocs.io/en/stable/writing-tests.html

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Kent Gibson <warthog618@gmail.com>