do not specify implementation dependant exception class names in fiber test

mrbgems/mruby-fiber/test/fiber.rb

@@ -42,25 +42,23 @@ assert('Fiber with splat in the block argument list') {
 }
 
 assert('Fiber raises on resume when dead') {
-  e2 = nil
   r1 = true
   begin
     f = Fiber.new{}
     f.resume
     r1 = f.alive?
     f.resume
+    false
   rescue => e1
-    e2 = e1
+    true
   end
-  r1 == false and e2.class == RuntimeError
 }
 
 assert('Yield raises when called on root fiber') {
-  e2 = nil
   begin
     Fiber.yield
+    false
   rescue => e1
-    e2 = e1
+    true
   end
-  e2.class == ArgumentError
 }