Bluetooth: Move re-implementation of `snprintk`

Move the function in the `subsys/testsuite/ztest/src/ztest_mock.c` files.

This is motivated by the fact that there is others re-implementation of
`*printk` functions using libc counterparts in the `ztest_mock.c` file.

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