messages: Add tests for transform_content.

[rsashank]

What does this PR do, and why?

This update PR tests for the transform_content class method for self.message_links and self.time_mentions. The addition of these tests addresses feedback from issue #1455, which pointed out the lack of tests for these aspects.

@neiljp said:

It'd be good to see some tests for what ends up in self.code_snippets, though we don't seem to have tests for the others.

External discussion & connections

• [ ] Discussed in #zulip-terminal in topic
• [ ] Fully fixes #
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] Adapting existing automated tests
• [x] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [] It contains test additions for any new behavior
• [x] It flows clearly from a previous branch commit, and/or prepares for the next commit