drivers: counter: rpi_pico_timer: Correct support for RP2350

Changes introduced in 293b9a16 didn't
verify correct behaviour when running `TIMER1` with `TIMER0` is paused,
and was reporting the wrong tick counter being incremented. Correct this
and extend the testing to verify correct behaviour.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>