[Platform] [mbed] Re-enable Unit Testing Firmware for mbed

[Alami-Amine]

Context

• since the PR #15966, Unit Testing for Mbed platform was disabled because the unit-testing firmware stopped fitting in the flash (all unit tests are compiled into one big binary).

• during the migration of unit testing framework from nl_unit to pw_unit_test framework, Mbed platform was not migrated to pigweed as there is no way for testing.

• This was discussed in:

Is there an github issues to track to re-enable this after we split tests to multiple smaller binaries? If so please include a TODO with the issue number. If not, please create one and add the issue number here

Originally posted by @tehampson in #33943 (comment)

Actions

• After splitting unit tests to multiple smaller binaries, it should be possible re-enable unit testing for Mbed and to test that.
• However, there is no platform maintainer for Mbed; therefore, the feasibility of this issue can be questioned.

Step to Re-enable:

1. Add support for running pw_unit_test in the mbed Test_driver: src/test_driver/mbed/unit_tests/main/main.cpp
2. Enable it in the Github Actions workflow:

   connectedhomeip/.github/workflows/examples-mbed.yaml

   Line 162 in 4cdce52

   if: false