CI failed on ruby 2.4 or earlier

[znz]

The log pointed out that exception_details is missing.
The line does not reach on 2.5 or later, but the problem exists too.
When 2.4 or earlier, undefined method `full_message' for #<RuntimeError: ok>\nDid you mean? message` raised at

fileutils/test/lib/core_assertions.rb

Line 381 in ff35d69

err.full_message(highlight: false, order: :top)

.
It causes the test failures.

https://github.com/ruby/fileutils/runs/254688215#step:5:9

===============================================================================
Error: test_assert_output_lines(TestFileUtils): NoMethodError: undefined method `exception_details' for #<TestFileUtils:0x000055abb296cba8>
/home/runner/work/fileutils/fileutils/test/lib/core_assertions.rb:216:in `block in assert_raise'
/home/runner/work/fileutils/fileutils/test/lib/core_assertions.rb:215:in `rescue in assert_raise'
/home/runner/work/fileutils/fileutils/test/lib/core_assertions.rb:226:in `assert_raise'
/home/runner/work/fileutils/fileutils/test/fileutils/test_fileutils.rb:236:in `test_assert_output_lines'
     233:   #
     234: 
     235:   def test_assert_output_lines
  => 236:     assert_raise(MiniTest::Assertion) {
     237:       Timeout.timeout(0.5) {
     238:         assert_output_lines([]) {
     239:           Thread.current.report_on_exception = false
===============================================================================