Cellular: Disable Greentea tracing to consume less memory

[AriParkkila]

Description

Cellular Greentea tests were failing with UBLOX_C027 target + IAR toolchain due to out-of-memory. This change disables tracing while running Greentea tests to consume less memory.

Fixes ARM internal ref IOTCELL-1082.

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[AriParkkila]

@mirelachirica @jarvte @kimlep01 please review

[kjbracey]

Sadly, tracing can occasionally break Greentea comms in general - it's not totally robust, unlike Icetea. Might want to do this more generally if any other of your tests have tracing enabled.

[0xc0170]

/morph build

[kimlep01]

Tests are passing now in CI

[mbed-ci]

Build : SUCCESS

Build number : 2315
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7182/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1938
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7182/

[mbed-ci]

Test : SUCCESS

Build number : 2094
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7182/2094