Improve leaked thread detector

While I was chasing down a leaked thread, I gathered a few
improvements. These mostly affect tests that finish too fast, my
test case was:

```ruby
RSpec.describe "A short-lived thread" do
  it 'lives for a really short while' do
    Thread.new { }
  end
end
```

Improvements:

* Set the `@caller` before the thread actually starts running. This
  works fine (we're using a similar construct in the profiler), and
  ensures that the `@caller` is available from the moment the thread
  gets created, instead of being a race.

  I observed that if a test run too fast, it was possible that
  `@caller` was not set because the thread still hadn't run by the
  time the detector started working.

* Call `Thread.pass` whenever a backtrace is not available -- this
  tries to give newly-created threads a chance to run, so that we
  can have a backtrace for them.

* Detect when a backtrace is missing because a thread has died.

spec/spec_helper.rb

@@ -129,8 +129,21 @@
         end
 
         info = background_threads.each_with_index.flat_map do |t, idx|
+          backtrace = t.backtrace
+          if backtrace.nil? && t.alive? # Maybe the thread hasn't run yet? Let's give it a second chance
+            Thread.pass
+            backtrace = t.backtrace
+          end
+          if backtrace.nil? || backtrace.empty?
+            backtrace =
+              if t.alive?
+                ['(Not available. Possibly a native thread.)']
+              else
+                ['(Thread finished before we could collect a backtrace)']
+              end
+          end
+
           caller = t.instance_variable_get(:@caller) || ['(Not available. Possibly a native thread.)']
-          backtrace = t.backtrace || ['(Not available. Possibly a native thread.)']
           [
             "#{idx + 1}: #{t} (#{t.class.name})",
             'Thread Creation Site:',
@@ -174,9 +187,8 @@
 # back to their creation point.
 module DatadogThreadDebugger
   def initialize(*args)
-    caller_ = caller
+    @caller = caller
     wrapped = lambda do |*thread_args|
-      Thread.current.instance_variable_set(:@caller, caller_)
       yield(*thread_args)
     end
     wrapped.ruby2_keywords if wrapped.respond_to?(:ruby2_keywords, true)