Fix SocketAddress unittests

[SeppoTakalo]

Description

Add ip4tos() and stoip4() into SocketAddress unittests.
NOTE: Probably should have been stubbed but this way we can also
test these helper functions.

#6293 Added usage of these helper functions, but by that time unit tests were not in place.

CC: @OPpuolitaival @kjbracey-arm

Pull request type

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

[OPpuolitaival]

@lorjala please review

[lorjala]

Looks good, but is identical to this #8018

Which one we choose? This one implements actual headers.

[SeppoTakalo]

Oh...
I would actually pull in the actual code into the test.
But I know some people would prefer dummy stubs that do nothing.

I prefer to pull in library code into unittests, so that the test can actually do something useful.

[cmonr]

@SeppoTakalo @OPpuolitaival @kjbracey-arm @lorjala

If I understand, are we going to prefer this PR over #8018?

[lorjala]

@cmonr we're going with this.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3066
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8086/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2678
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8086/

[adbridge]

/morph test

[cmonr]

/morph test

[cmonr]

Trying that again...
/morph test

[mbed-ci]

Test : FAILURE

Build number : 2846
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8086/2846

[cmonr]

Stopped job to help root cause network issue.

[adbridge]

Kicking off again to see if we have more luck this morning!
/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2855
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8086/2855