tests/sys/shell: use default terminal instead of socat

[mguetschow]

Contribution description

tests/sys/shell currently per default uses socat as terminal, according to the comment to avoid having extra characters put into the IO by the terminal program. But that's exactly what make cleanterm is for, which is already used per default by the python testrunner.

I encountered issues with socat for this test (newlines not properly matched against) on boards that use usb_cdc_acm as stdio, while the approach with cleanterm works as expected.

Testing procedure

Run the test with some boards using make -C tests/sys/shell BOARD=xxx flash test. Successfully tested with native, nrf52840dk and feather-nrf52840-sense.

Issues/PRs references

The commit cdfece6 that introduced the change was merged one and a half months before the introduction of the cleanterm target in #12107