tests: bsim: Use the pre_init_f instead of post_init_f

When using the post_init_f to initialize the `bst_result`,
it is not possible to mark the test as
passed immediately as the `bst_result` will be
initialized after the test completes.

This change should overcome this limitation.

Bluetooth mesh tests are kept as is as we are not
sure if this will change the behavior.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>