testimage: create a list of failed test post actions

testimage: create a list of failed test post actions

testimage is able to detect whenever a test run leads to some tests
failing, and execute some actions in this case. The only action currently
defined in such case is to retrieve artifacts from the target under test,
as listed in TESTIMAGE_FAILED_QA_ARTIFACTS

In order to be able to add multiple actions, define a central function to
gather all "post actions" to run whenever a test has failed
(run_failed_tests_post_actions). This function contains a table listing all
functions to be called whenever a test fails. Any function in this table
will be provided with bitbake internal data dictionary ("d") and the
current runtime testing context ("tc"). Isolate all this feature in a
dedicated postactions.py file inherited by testimage.
This patch does not bring any functional change.

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>