test/ruby/test_fiber.rb: Skip the problematic test_stack_size on riscv

Since it is impossible to detect stack overflow of C functions robustly,
the test is a bit unreasonable.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/debian-riscv64/ruby-master/log/20200421T120018Z.fail.html.gz

test/ruby/test_fiber.rb

@@ -361,6 +361,7 @@ class TestFiber < Test::Unit::TestCase
 
   def test_stack_size
     skip 'too unstable on Travis s390x-linux' if RUBY_PLATFORM == 's390x-linux'
+    skip 'too unstable on riscv' if RUBY_PLATFORM =~ /riscv/
     h_default = eval(invoke_rec('p RubyVM::DEFAULT_PARAMS', nil, nil, false))
     h_0 = eval(invoke_rec('p RubyVM::DEFAULT_PARAMS', 0, 0, false))
     h_large = eval(invoke_rec('p RubyVM::DEFAULT_PARAMS', 1024 * 1024 * 5, 1024 * 1024 * 10, false))