channels: Don't rely on tight timing constraints (JuliaLang#49090)

The test was relying on two particular non-guaranteed timing races:

1. That the 0.5s wait task was started within 0.5s of the timedwait
   starting.
2. That the timedwait finished within 0.5 seconds of its deadline.

Neither of these are guaranteed. The second of these was already
made non-fatal with a warning print. However, I don't like warning
prints in tests. They should either pass or fail. Nobody looks at
the output unless the tests fail, so change the tests to:

1. Add extra synchronization to solve the race condition (1) by
making sure the sleep starts before the timedwait.
2. Instead of doing a long wait, just have a synchronization channel
that will never be made active.

In particular, the thing that this test checks is that the timeout
in `timedwait` actually works, and doesn't return too early. This
arrangement checks both of those properties without unduly tight
timing constraints.

test/channels.jl

@@ -311,6 +311,7 @@ end
 
 @testset "timedwait on multiple channels" begin
     Experimental.@sync begin
+        sync = Channel(1)
         rr1 = Channel(1)
         rr2 = Channel(1)
         rr3 = Channel(1)
@@ -320,20 +321,17 @@ end
         @test !callback()
         @test timedwait(callback, 0) === :timed_out
 
-        @async begin sleep(0.5); put!(rr1, :ok) end
+        @async begin put!(sync, :ready); sleep(0.5); put!(rr1, :ok) end
         @async begin sleep(1.0); put!(rr2, :ok) end
-        @async begin sleep(2.0); put!(rr3, :ok) end
+        @async begin @test take!(rr3) == :done end
 
+        @test take!(sync) == :ready
         et = @elapsed timedwait(callback, 1)
 
-        # assuming that 0.5 seconds is a good enough buffer on a typical modern CPU
-        try
-            @assert (et >= 1.0) && (et <= 1.5)
-            @assert !isready(rr3)
-        catch
-            @warn "`timedwait` tests delayed. et=$et, isready(rr3)=$(isready(rr3))"
-        end
+        @test et >= 1.0
+
         @test isready(rr1)
+        put!(rr3, :done)
     end
 end