spec/ruby/core/io/*_spec.rb: explain MJIT failures

These random failures happen because FD number allocation
is not predictable when multiple threads are running (since
MJIT thread is opening/closing files all the time).

Real-world code practically never relies on predictable FD
number allocation (because much real-world code is
multi-threaded); so it's highly unlikely there'll be any
breakage to the user.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66159 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/io/initialize_spec.rb

@@ -13,9 +13,9 @@ describe "IO#initialize" do
     rm_r @name
   end
 
-  # http://ci.rvm.jp/results/trunk-mjit@silicon-docker/1469621
-  # http://ci.rvm.jp/results/trunk-mjit@silicon-docker/1454818
-  without_feature :mjit do # with RubyVM::MJIT.enabled?, this randomly fails for now
+  # File descriptor numbers are not predictable in multi-threaded code;
+  # MJIT will be opening/closing files the background
+  without_feature :mjit do
     it "reassociates the IO instance with the new descriptor when passed a Fixnum" do
       fd = new_fd @name, "r:utf-8"
       @io.send :initialize, fd, 'r'

spec/ruby/core/io/reopen_spec.rb

@@ -145,9 +145,9 @@ describe "IO#reopen with a String" do
     File.read(@other_name).should == "new data"
   end
 
-  # http://ci.rvm.jp/results/trunk-mjit@silicon-docker/1461550
-  # http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/1448152
-  without_feature :mjit do # with RubyVM::MJIT.enabled?, this randomly fails for now
+  # File descriptor numbers are not predictable in multi-threaded code;
+  # MJIT will be opening/closing files the background
+  without_feature :mjit do
     it "closes the file descriptor obtained by opening the new file" do
       @io = new_io @name, "w"