Merge the tls_test.c into netmgr_test.c and extend the tests suite

This merge request combines TLS tests (tls_test.c) into the common Network Manager unit tests suite (netmgr_test.c) and extends the unit test framework to include support for additional "ping-pong" style tests where all data could be sent via lesser number of connections (the behaviour of the old test suite). The tests for TCP and TLS were extended to make use of the new mode, as this mode better translates to how the code is used in DoH.

Both TLS and TCP tests now share most of the unit tests' code, as they are expected to function similarly from a users's perspective anyway.

Additionally to the above, the TLS test suite was extended to include TLS tests which use the active connections quota facility.