Do not assume the next fd is always free in Process.exec specs

* It might be useful by something else until the execve(2) call.

Author: eregon

spec/ruby/core/process/exec_spec.rb

@@ -193,9 +193,11 @@
           map_fd_fixture = fixture __FILE__, "map_fd.rb"
           cmd = <<-EOC
             f = File.open(#{@name.inspect}, "w+")
-            child_fd = f.fileno + 1
-            File.open(#{@child_fd_file.inspect}, "w") { |io| io.print child_fd }
-            Process.exec "#{ruby_cmd(map_fd_fixture)} \#{child_fd}", { child_fd => f }
+            File.open(#{__FILE__.inspect}, "r") do |io|
+              child_fd = io.fileno
+              File.open(#{@child_fd_file.inspect}, "w") { |io| io.print child_fd }
+              Process.exec "#{ruby_cmd(map_fd_fixture)} \#{child_fd}", { child_fd => f }
+            end
             EOC
 
           ruby_exe(cmd, escape: true)