tests: drivers: uart_emul: extend test case for async API

- Tests that the API of the uart-emul also works with the async API
- Tests that `uart_emul_set_release_buffer_on_timeout()` configures the
  behavior as expected
- The tests are now splited into three different configurations: only
  polling API enabled, additionally interrupt driven API enabled and
  additionally async API enabled
- Enabled `native_sim` board

Signed-off-by: Ruben Völl <ruben.voell@grandcentrix.net>