scripts: Make workflow of test_plan.py script more robust

The script was not resolving all detected changes uniformly:
find_excludes() could skip or not certain patterns based on
required testing scope. The idea was to not include files
that were already handled by find_test() and find_boards() workflows.
However, only boards and tests folders could be removed but
not samples. This also led to blind spots: changes in some files
were not triggering any tests. E.g. change in a test/common, where
no corresponding yaml can be found by find_tests() which is also
ignored by find_excludes(). In the new workflow a list of resolved
files (for which find_arch(), find_tests() or find_boards()
found scope) is created. Instead of using skip in find_excludes,
files are excluded only if they were resolved.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>