tests: nrf: enable twister tests for nrf54l20pdk

Enabled the following tests on nrf54l20
- i2c_slave
- clock_control_api
- nrf_clock_calibration
- nrf_lf_clock_start
- onoff
- retained_mem/api
- uart_elementary
- uart_errors
- uart_mix_fifo_poll
- uart_pm
- uart_async_api
- wdt_error_cases
- wdt_basic_api
- counter_basic_api
- fcb
- nrf/qdec

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>