Fix CI build errors and warnings.

mrbgems/mruby-bin-mirb/bintest/mirb.rb

@@ -12,8 +12,8 @@ assert('regression for #1563') do
 end
 
 assert('mirb -d option') do
-  o, s = Open3.capture2('bin/mirb', :stdin_data => "p $DEBUG\n")
+  o, _ = Open3.capture2('bin/mirb', :stdin_data => "p $DEBUG\n")
   assert_true o.include?('=> false')
-  o, s = Open3.capture2('bin/mirb -d', :stdin_data => "p $DEBUG\n")
+  o, _ = Open3.capture2('bin/mirb -d', :stdin_data => "p $DEBUG\n")
   assert_true o.include?('=> true')
 end

mrbgems/mruby-bin-mruby/bintest/mruby.rb

@@ -60,8 +60,8 @@ RUBY
 end
 
 assert('mruby -d option') do
-  o = `#{cmd('mruby')} -e #{shellquote('p $DEBUG')}>&1`
-  assert_equal o, "false\n"
-  o = `#{cmd('mruby')} -d -e #{shellquote('p $DEBUG')}>&1`
-  assert_equal o, "true\n"
+  o = `#{cmd('mruby')} -e #{shellquote('p $DEBUG')}`
+  assert_equal "false\n", o
+  o = `#{cmd('mruby')} -d -e #{shellquote('p $DEBUG')}`
+  assert_equal "true\n", o
 end