Move pread + pwrite tests out of RUBY_ENGINE block

These tests were guarded by a RUBY_ENGINE of "ruby" even though they test an official Ruby feature (pread/pwrite added in Ruby 2.5). This commit moves them to the top level of the test case so they will run on other implementations.

test/ruby/test_io.rb

@@ -3813,29 +3813,29 @@ __END__
         end
       end
     end
-
-    def test_pread
-      make_tempfile { |t|
-        open(t.path) do |f|
-          assert_equal("bar", f.pread(3, 4))
-          buf = "asdf"
-          assert_equal("bar", f.pread(3, 4, buf))
-          assert_equal("bar", buf)
-          assert_raise(EOFError) { f.pread(1, f.size) }
-        end
-      }
-    end if IO.method_defined?(:pread)
-
-    def test_pwrite
-      make_tempfile { |t|
-        open(t.path, IO::RDWR) do |f|
-          assert_equal(3, f.pwrite("ooo", 4))
-          assert_equal("ooo", f.pread(3, 4))
-        end
-      }
-    end if IO.method_defined?(:pread) and IO.method_defined?(:pwrite)
   end
 
+  def test_pread
+    make_tempfile { |t|
+      open(t.path) do |f|
+        assert_equal("bar", f.pread(3, 4))
+        buf = "asdf"
+        assert_equal("bar", f.pread(3, 4, buf))
+        assert_equal("bar", buf)
+        assert_raise(EOFError) { f.pread(1, f.size) }
+      end
+    }
+  end if IO.method_defined?(:pread)
+
+  def test_pwrite
+    make_tempfile { |t|
+      open(t.path, IO::RDWR) do |f|
+        assert_equal(3, f.pwrite("ooo", 4))
+        assert_equal("ooo", f.pread(3, 4))
+      end
+    }
+  end if IO.method_defined?(:pread) and IO.method_defined?(:pwrite)
+
   def test_select_exceptfds
     if Etc.uname[:sysname] == 'SunOS' && Etc.uname[:release] == '5.11'
       skip "Solaris 11 fails this"