On-PR CI needs to build a subset of tests for a subset of platforms regardless of the scope of the PR changes

[carlescufi]

There are some tests that are required to be built for a certain set of platforms in order to verify that a set of changes of changes in a PR has not broken the code.
As an example, we can use tests/bluetooth/init. In those tests, there is a subset with a platform_allow set to a few Nordic platforms. For every PR (or at the very least whenever code in subsys/bluetooth is modified), those tests should be built for those nRF platforms. This is not the case today:

$ scripts/sanitycheck --integration -T tests/bluetooth/init/
INFO    - Zephyr version: zephyr-v2.4.0-1254-gfbc487cce3
INFO    - JOBS: 8
INFO    - Selecting default platforms per test case
INFO    - Building initial testcase list...
INFO    - 40 test configurations selected, 11920 configurations discarded due to filters.
INFO    - Adding tasks to the queue...

INFO    - 0 of 0 tests passed (0.00%), 0 failed, 40 skipped with 0 warnings in 0.61 seconds
INFO    - In total 0 test cases were executed on 1 out of total 299 platforms (0.33%)
INFO    - 0 tests executed on platforms, 0 tests were only built.

Another example is building the MCUBoot tests for nRF platforms to avoid regressions, this is documented in #29397

[carlescufi]

CC @trond-snekvik @cvinayak @nvlsianpu @joerchan

[nashif]

changing to a bug, at some point this used to work as expected here.

[PerMac]

I have an idea for a solution for workflows where we want to be sure that building is happening on selected platforms:
-add the required platform to integration_platforms list in tests' yaml. However, this will need additional changes in the framework.
The idea is that integration_platforms list in tests' yamls is used to list the platforms to which testing will be limited when -G (--integration) arg is used for sanitycheck (it is used by the CI). However, currently, the first check is if the platform has testing: default: true in its board.yaml. If not, even if the platform is in the integration list for the test, it is skipped when -G is used. I wonder if it won't be better that when the platform is in the integration_platforms in a test's yaml the test will be called regardless of testing: default: true in the board.yaml. What do you think about such workflow?