Fix bad await loop in tracer test

[marcotc]

The following wait loop does not exist as one would expect:

1000.times do
  mutex.synchronize do
    break unless @thread_ctx.nil? || @thread_span.nil?
  end
  sleep 0.01
end

The break statement is actually breaking out of #synchronize, not #times, meaning this test always runs to its maximum time of around 11s.

This PR replaces this loop with our helper method try_wait_until. I also removed the mutex around the instance variable assignment verification, as instance variable assignment is an atomic VM operation.

This is the slowest single test in our test suite currently.

[delner]

Nice! I was wondering what test took so long in the Minitest suite.