Cellular: Fixed sms unit tests.

[jarvte]

Description

Fixed Cellular SMS unit tests. No code changes.

ARM internal ref: IOTCELL-582

@AnttiKauppila please review.

Pull request type

• Fix
• Refactor
• New target
• Feature
• Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1441
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6345/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1087
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6345/

[mbed-ci]

Test : SUCCESS

Build number : 1221
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6345/1221