Tests: bsim: Use correct compilation folder

If a test was written outside 'app_root' the 'app' path may contain
previous directory ('..') which would lead to the compilation file being
written outside the 'WORK_DIR'.

Fix the issue by checking if the 'app' is inside 'app_root', if it's
not, use the absolute path to the app for creating the 'this_dir'
variable.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>