test: fix small race in AsyncCondition test (JuliaLang#39583)

There was a small race window where the second t might not have finished
exiting, leading to interference in the Workqueue, resulting in
process_events from running again before the async callback had a chance
to run and dequeue the first event. (This test is probably much stronger
than necessary.)

N.B. This depends on the recent changes in
af353ec for being able to extract the
internal state needed for the sake of the test.

Fixes JuliaLang#39527

test/channels.jl

@@ -390,36 +390,43 @@ end
  t = Timer(0) do t
  tc[] += 1
  end
+ cb = first(t.cond.waitq)
  Libc.systemsleep(0.005)
  @test isopen(t)
  Base.process_events()
  @test !isopen(t)
  @test tc[] == 0
  yield()
  @test tc[] == 1
+ @test istaskdone(cb)
  end
 
  let tc = Ref(0)
  t = Timer(0) do t
  tc[] += 1
  end
+ cb = first(t.cond.waitq)
  Libc.systemsleep(0.005)
  @test isopen(t)
  close(t)
  @test !isopen(t)
- sleep(0.1)
+ wait(cb)
  @test tc[] == 0
+ @test t.handle === C_NULL
  end
 
  let tc = Ref(0)
  async = Base.AsyncCondition() do async
  tc[] += 1
  end
+ cb = first(async.cond.waitq)
  @test isopen(async)
  ccall(:uv_async_send, Cvoid, (Ptr{Cvoid},), async)
  ccall(:uv_async_send, Cvoid, (Ptr{Cvoid},), async)
+ @test isempty(Base.Workqueue)
  Base.process_events() # schedule event
  Sys.iswindows() && Base.process_events() # schedule event (windows?)
+ @test length(Base.Workqueue) == 1
  ccall(:uv_async_send, Cvoid, (Ptr{Cvoid},), async)
  @test tc[] == 0
  yield() # consume event
@@ -440,13 +447,16 @@ end
  yield() # consume event & then close
  @test tc[] == 3
  sleep(0.1) # no further events
+ wait(cb)
  @test tc[] == 3
+ @test async.handle === C_NULL
  end
 
  let tc = Ref(0)
  async = Base.AsyncCondition() do async
  tc[] += 1
  end
+ cb = first(async.cond.waitq)
  @test isopen(async)
  ccall(:uv_async_send, Cvoid, (Ptr{Cvoid},), async)
  Base.process_events() # schedule event
@@ -457,8 +467,10 @@ end
  @test tc[] == 0
  yield() # consume event & then close
  @test tc[] == 1
- sleep(0.1)
+ sleep(0.1) # no further events
+ wait(cb)
  @test tc[] == 1
+ @test async.handle === C_NULL
  end
 end