Unstable TestRDocGeneratorJsonIndex#test_generate test

I am seeing the following failure that happens randomly on Travis Ubuntu jammy ppc64le environment in ruby/ruby. The latest failure is at the relatively recent commit https://github.com/ruby/ruby/commit/1d51e4cadfbc1f445d00887189e06047759c9aec .

https://app.travis-ci.com/github/ruby/ruby/jobs/612435804#L2936

  1) Failure:
TestRDocGeneratorJsonIndex#test_generate [/home/travis/build/ruby/ruby/test/rdoc/test_rdoc_generator_json_index.rb:108]:
.js files should be the same timestamp of original

The test is built and executed with the following steps.

+ export NPROC="$(nproc)"
+ export JOBS=-j$((1+${NPROC}))
+ export RUBY_TESTOPTS="$JOBS -q --tty=no"
+ export optflags=-O1
+ export debugflags=-ggdb3

+ export CC=gcc-11
+ gcc-11 --version
gcc-11 (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ../tool/travis_wait.sh $SETARCH make -s test-all RUBYOPT="-w"
...
Run options: 
  --seed=2200
  "--ruby=./miniruby -I../lib -I. -I.ext/common  ../tool/runruby.rb --extout=.ext  -- --disable-gems"
  --excludes-dir=../test/.excludes
  --name=!/memory_leak/
  -j3
  -q
  --tty=no

Start with test/rdoc/test_rdoc_generator_json_index.rb at line 108 and inspect the timestamp assertion in TestRDocGeneratorJsonIndex#test_generate. Reproduce under the reported Travis Ubuntu jammy ppc64le settings with make test-all and the shown RUBY_TESTOPTS, then determine why the generated .js timestamp differs intermittently. Done means the test reliably passes while still checking the expected timestamp.