tests: drivers: can: add a delay for the shell backend to initialize

The CAN shell test is flaky on some qemu platform (qemu_riscv64_smp in
particular), where the first test command occasionally fails as it tries
to run with the backend not initialized.

The tests for subsys/shell/shell/src/main.c have a setup delay, copying
it to the CAN shell test seems to make it work reliably.

Tested with:
  ./scripts/twister -v -n -M -p qemu_riscv64_smp -T tests/drivers/can/shell

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>