test/ruby/test_fiber.rb (test_many_fibers_with_threads): relax timeout

This test takes 40..50 seconds under Solaris 11, so the timeout (60
seconds) was too strict.  This change increases it to 180 seconds.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/solaris11-gcc/ruby-master/log/20200322T100007Z.fail.html.gz
```
  1) Error:
TestFiber#test_many_fibers_with_threads:
Timeout::Error: execution of assert_normal_exit expired timeout (60 sec)
```

test/ruby/test_fiber.rb

@@ -50,7 +50,7 @@ class TestFiber < Test::Unit::TestCase
   end
 
   def test_many_fibers_with_threads
-    assert_normal_exit <<-SRC, timeout: 60
+    assert_normal_exit <<-SRC, timeout: 180
       max = 1000
       @cnt = 0
       (1..100).map{|ti|