Cellular: Fix Greentea test for network attach

[AriParkkila]

Description

Added 10 seconds wait in Greentea test before trying to attach after network registration test. Cellular network attach can't be done before DUT is registered to a network.

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[AriParkkila]

@jarvte please review

[AnttiKauppila]

This is targeted to 5.10.0-rc2

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2993
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7910/

Triggering tests

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

[mbed-ci]

Test : FAILURE

Build number : 2756
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7910/2756

[adbridge]

/morph test

[adbridge]

Restarted tests on this PR as the failure did not look to be related

[mbed-ci]

Exporter Build : FAILURE

Build number : 2604
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7910/

[mbed-ci]

Test : SUCCESS

Build number : 2760
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7910/2760

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2612
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7910/