test: better organize source test with different encodings

[xxchan]

• [x] We already implemented a better style https://github.com/risingwavelabs/risingwave/pull/17434
• [ ] cleanup existing tests, and only leave a few meaningful e2e tests.

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Currently test data looks like this:

Besides, there's actually another folder: src/connector/src/test_data

It's way too messy:

• The format has different conventions.
• We don't know how they are ingested. (There are multiple commands/scripts!)
• Actually, the filename has a convention, and will decide the behavior. https://github.com/risingwavelabs/risingwave/blob/7a16a2c51b019b73163e53d855639ebd11a0b2f6/scripts/source/prepare_ci_kafka.sh#L49-L66

The result is that even experienced developers (@xiangjinwu) need to relearn the convention each time. However, we are still developing many connector features, and the difficulty of adding new tests will hinder development.

Some ideas for improvement:

• For the very basic requirement, we should group test data into folders by how they are used, instead of relying on filename suffix (and has a README for how to use them..)

[xxchan]

With https://github.com/risingwavelabs/risingwave/issues/17002, we can simply do more unit tests instead of e2e tests.

We can also develop web apps https://risingwave-labs.slack.com/archives/C03A2PSS8KU/p1717127312643039

We can also do fuzz tests