Essentially this reverts 45464bfcbdf9f9cfb440950bc57a27d237627a17.
The commit removed a mock of Time.now, which caused a random failure.
http://rubyci.s3.amazonaws.com/ubuntu1804/ruby-master/log/20210512T123004Z.fail.html.gz
```
1) Failure:
TestGemPackageTarWriter#test_add_file_signer [/home/chkbuild/chkbuild/tmp/build/20210512T123004Z/ruby/test/rubygems/test_gem_package_tar_writer.rb:117]:
Field mtime of the tar header differs..
<"14046746312\u0000"> expected but was
<"14046746311\x00">.
```
Object#stub is defined at f1af59fe02ef2cc58f13e2742e4cc6cf8c2a1a20, so
now `Time.stub :now` works.
https://github.com/rubygems/rubygems/commit/85f60a9ed0