[lldb] Fix flakyness in TestProcessList

If the test is too fast it can read the process list before the forked
child process actually manages to exec the process with the right
arguments.

Use our file-based synchronization primitives to ensure the child is
up-and-running before we fetch the process list.

GitOrigin-RevId: b53641cb72acae8973a16c4f5567885bd3fe85c0
2 files changed