docs: more details about testing (#3856)

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Author: IWANABETHATGUY

docs/contrib-guide/testing.md

@@ -6,6 +6,13 @@ run `just test-update` to run all tests and update snapshots automatically
 
 We have two groups of test suites: one for Rust, and one for Node.js.
 
+:::warning Test principle you should respect
+
+1. When adding new feature with options, always make sure adding related tests in javascript side if possible.
+
+Some test related skills maybe helpful [details](#test-skills)  
+:::
+
 ## Summary
 
 - `just test` for running all tests.
@@ -109,3 +116,24 @@ In `/packages/rollup-tests`:
 
 - `just test-node rollup` will run rollup tests.
 - `just test-node rollup --update` will run and update the tests' status.
+
+### Test skills
+
+Our rust test infra is powerful enough to cover most of the case of javascript(plugin, passing function inside config).
+But since Javascript side user is still our first class user, try to put tests in javascript side if possible.
+Here are some experience about what test technique you should use.
+:::tip TLDR
+Add test in javascript side if you don't want to wasting time on deciding which way to use.
+:::
+
+#### Prefer Rust
+
+1. Test warning or error emitted by rolldown core.
+   - [error](https://github.com/rolldown/rolldown/blob/568197a06444809bf44642d88509313ee2735594/crates/rolldown/tests/rolldown/errors/assign_to_import/artifacts.snap?plain=1#L2-L54)
+   - [warning](https://github.com/rolldown/rolldown/blob/568197a06444809bf44642d88509313ee2735594/crates/rolldown/tests/rolldown/warnings/eval/artifacts.snap?plain=1#L1-L28)
+2. Matrix testing, assume you want to test a suite different [format](https://github.com/rolldown/rolldown/blob/568197a06444809bf44642d88509313ee2735594/crates/rolldown/tests/rolldown/topics/bundler_esm_cjs_tests/4/_config.json?plain=1#L1-L21), with `configVariants` you could do that with only one test.
+3. Tests related to linking algorithm(tree shaking, chunk splitting) Those may require a lot of debugging, add test on rust side could reduce the time of coding-debug-coding work loop.
+
+#### Prefer Javascript
+
+Any category not mentioned above should put in javascript side.