Ensure sending fiber finished in select specs (#8724)

spec/std/concurrent/select_spec.cr

@@ -114,7 +114,7 @@ describe "select" do
     ch3 = Channel(Int32).new
     x = nil
 
-    spawn do
+    f = spawn do
       select
       when x = ch1.receive
       when x = ch2.receive
@@ -128,7 +128,9 @@ describe "select" do
     end
 
     ch3.receive.should eq(3)
-    Fiber.yield
+    until f.dead?
+      Fiber.yield
+    end
     x.should eq(1)
   end
 
@@ -138,7 +140,7 @@ describe "select" do
     ch3 = Channel(Int32).new
     x = nil
 
-    spawn do
+    f = spawn do
       select
       when ch1.send 1
         x = 1
@@ -154,7 +156,9 @@ describe "select" do
     end
 
     ch3.receive.should eq(3)
-    Fiber.yield
+    until f.dead?
+      Fiber.yield
+    end
     x.should eq(1)
   end