Drop MBED_EXTENTED_TESTS flagging from network tests.

[SeppoTakalo]

Description

All properly working IP stacks are required to pass Mbed OS socket tests.
This have now proven with current Ethernet devices with LwIP stack.

Test results: From internal Jenkins when running full set of network tests on Mbed OS:

Pull request type

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

[SeppoTakalo]

@OPpuolitaival @VeijoPesonen Please review.

[cmonr]

CI started

[cmonr]

@SeppoTakalo I'm going to restart greentea tests a couple of times since this enablement has burned us in the past.

Will hold off on merging until @OPpuolitaival or someone from @ARMmbed/mbed-os-test-team gives the OK.

[cmonr]

Fingers crossed that the CI's networking backend takes kindly to this test.

[mbed-ci]

Test run: SUCCESS

Summary: 9 of 9 test jobs passed
Build number : 1
Build artifacts

[cmonr]

Second run passed just fine. Running once more, and should be good.

[OPpuolitaival]

These tests are important for Pelion project. I did check CI and error was not related to this. Restarted the failed board tests

[jeromecoutant]

Hi

You should then increase the test timeout value now?

Ex for DNS test, timeout value is 120

target	platform_name	test suite	result	elapsed_time (sec)	copy_method
DISCO_L475VG_IOT01A-ARM	DISCO_L475VG_IOT01A	tests-netsocket-dns	OK	123.94	default

[SeppoTakalo]

@jeromecoutant Please send a PR for longer timeout values.

There have originally been set optimistically for Ethernet, so some Cellular devices or external serial driven devices might not pass within the time.