support/testing: use .assertRunOk() when possible

The BRTest() class implements an assertRunOk() method that does the
very common work of running a command inside the emulator, and
checking that it is successful.

This commit changes all locations where this .assertRunOk() method can
be used, instead of open-coding the same logic.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>