discovery: fix a flaky test failure in mojo

  * Fix the global discovery scan to not return LOST event when the
    context is done
  * Avoid blocking when stopping by using a buffer channel in mojo
    discovery test

Fix https://github.com/vanadium/build/issues/63

MultiPart: 2/2
Change-Id: I2142518772d5a0c12296d2204ffe3ae4c6903be6
2 files changed