test/irb/test_raise_no_backtrace_exception.rb: UTF-8 is expected

... for the output of assert_in_out_err.

This will fix the following failure on many CI machines

http://rubyci.s3.amazonaws.com/arch/ruby-master/log/20210511T030005Z.fail.html.gz
```
  1) Failure:
TestIRB::TestRaiseNoBacktraceException#test_raise_exception_with_different_encoding_containing_invalid_byte_sequence [/home/chkbuild/chkbuild/tmp/build/20210511T030005Z/ruby/test/irb/test_raise_no_backtrace_exception.rb:41]:
pid 221531 exit 0.

1. [1/2] Assertion for "stdout"
   | invalid byte sequence in US-ASCII.
```

test/irb/test_raise_no_backtrace_exception.rb

@@ -38,7 +38,7 @@ def raise_euc_with_invalid_byte_sequence
             end
           EOF
         end
-        assert_in_out_err(bundle_exec + %w[-rirb -W0 -e ENV[%(LC_ALL)]=%(ja_JP.UTF-8) -e ENV[%(LANG)]=%(ja_JP.UTF-8) -e IRB.start(__FILE__) -- -f --], <<~IRB, /`raise_euc_with_invalid_byte_sequence': あ\\xFF \(RuntimeError\)/, [])
+        assert_in_out_err(bundle_exec + %w[-rirb -W0 -e ENV[%(LC_ALL)]=%(ja_JP.UTF-8) -e ENV[%(LANG)]=%(ja_JP.UTF-8) -e IRB.start(__FILE__) -- -f --], <<~IRB, /`raise_euc_with_invalid_byte_sequence': あ\\xFF \(RuntimeError\)/, [], encoding: "UTF-8")
           require_relative 'euc'
           raise_euc_with_invalid_byte_sequence
         IRB