Compile `mcp` on Ubuntu 20.04

[evanmayer]

NOTE: These are changes that BREAK backwards compatibility. After this PR is approved, the changes I have made to mcp calls to the modbus API will no longer be compatible with libraries found on Debian 8 versions of the flight computers.

Apologies once again to all reviewers for another big PR. A lot of unit test development and build system improvements have been dammed behind moving to Ubuntu 20.04. There are a total of 48 new individual unit tests, bringing the current total to 76. Thankfully, this PR only touches a few areas of code, so it shouldn't be too cumbersome to review.

Change Summary

• Build system updates

  • CMakeLists.txt cleanup
  • CMake policy updates to be more compatible with newer CMake
  • Compiler flag updates to be compatible with newer linker
  • Debug mode compiles with -g3 to enable printing #define'd symbols in the debugger
  • NEW CMake option: run unit tests upon building the CMake target under test, ON by default
  • NEW CMake option: run gcov to get insight on code coverage metrics, OFF by default
  • CMake option: compile unit tests, now ON by default
  • GitHub action to build flight software on an Ubuntu 20.04 runner upon PR or push to main branch

• Code changes

  • Update modbus_set_response_timeout() function signature
  • Tests for actuators.c, and:
  • Formatting cleanup
  • Refector DoLock(): separate into two functions, GetLockAction() and DoLockAction() to simplify testing
  • Tests for balance.c, and:
  • Formatting cleanup
  • magnetometer.c formatting cleanup