Update Greentea EMAC tests

[juhaylinen]

Description

Increase timeout for EMAC tests. Old timeout (600 seconds) was not
enough for slower devices to complete the tests.

Check that connection was successful when running test case
emac_test_initialize.

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2921
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7880/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2540
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7880/

[mbed-ci]

Test : SUCCESS

Build number : 2672
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7880/2672