imap: update test certificate

This is a backport of the corresponding change in ruby2.5, where the
corresponding files were copied here.

The new test files were copied from the ones provided by the following
upstream commits:

https://github.com/ruby/ruby/commit/f234e6c3d3170f37508e214cdaef78d4b2584e5a
https://github.com/ruby/ruby/commit/1e0b49a293d3792826c67b7e05c5fcbd09c9ea6e

Closes: #919999