test/rdoc/test_rdoc_generator_json_index.rb: pend in test_generate in ppc64le.

[junaruga]

This PR is a workaround for the #1048 to always pass the test in ruby/ruby's Travis CI.

---

We observed that this test randomly fails in the ruby/ruby Travis ppc64le case. This commit is to pend the test_generate if the assertion for the generated file's modified time fails in a ppc64le environment.

Note that I didn't use the word "Travis CI" or Travis CI specific environment variables such as TRAVIS and TRAVIS_CPU_ARCH[1] in the code. Because I wanted to prioritize the rdoc's independence from the ruby/ruby.

[1] https://docs.travis-ci.com/user/environment-variables/#default-environment-variables

---

Note that RUBY_PLATFORM prints the following text in RubyCI's ppc64le Ubuntu server.

$ ruby -e 'puts RUBY_PLATFORM'
powerpc64le-linux-gnu

If the issue happens, the result of the test should be printed like this in ruby/rdoc repository. Note this is not actual log. I created it to explain it.

$ bundle exec rake test
...
Pending: test_generate(TestRDocGeneratorJsonIndex):
  Unstable test in ppc64le.
  <2023-10-30 15:38:35.936097453 +0100> expected but was
  <2023-10-30 15:38:35 +0100>.
/home/jaruga/var/git/ruby/rdoc/test/rdoc/test_rdoc_generator_json_index.rb:112:in `test_generate'
     109:     # https://github.com/ruby/rdoc/issues/1048
     110:     if orig_file.mtime.inspect != generated_file.mtime.inspect &&
     111:       RUBY_PLATFORM =~ /powerpc64le/
  => 112:       pend <<~EOC
     113:         Unstable test in ppc64le.
     114:         <#{orig_file.mtime.inspect}> expected but was
     115:         <#{generated_file.mtime.inspect}>.
...
2249 tests, 5715 assertions, 0 failures, 0 errors, 1 pendings, 0 omissions, 0 notifications
99.9555% passed
...

[junaruga]

All the tests passed. Let me merge the PR.