[netsocket][tests] missing documentation about tls testing.

[ithinuel]

Description

The Readme in mbed-os/TESTS/netsocket only presents the inetd's configuration for non-secured socket tests.
Would it be possible to add details on key and certificate creation and secured socket server (stunnel ?) setup ?

EDIT:
It might also be interesting to ease the update of the root certificate via a configurable header rather than editing the test's main.cpp living deep in mbed-os' TESTS subfolder.

Issue request type

[ ] Question
[x] Enhancement
[ ] Bug

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-955

[0xc0170]

cc @ARMmbed/mbed-os-ipcore

[michalpasztamobica]

Should be fixed with #11440