[doc] Add a basic doc explaining how to run Taichi CPP tests (#2502)

* [doc] Add a basic doc explaining how to run Taichi CPP tests

* Auto Format

* Update docs/lang/articles/contribution/writing_cpp_tests.md

Co-authored-by: Taichi Gardener <[email protected]>

docs/lang/articles/contribution/writing_cpp_tests.md

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+---
+sidebar_position: 11
+---
+
+# Workflow for writing a CPP test
+
+We strongly recommend each developer to write CPP unit tests when sending a PR.
+
+- We use [googletest](https://github.com/google/googletest) as the CPP
+  test infrastructure.
+- CPP tests should be added to the `tests/cpp/` directory.
+
+## Build and run Taichi CPP tests
+
+```bash
+# inside build/
+cmake .. -DTI_BUILD_TESTS=ON # ... other regular Taichi cmake args
+make
+
+# run the CPP test
+./taichi_cpp_tests
+```
+
+:::note
+Consider polishing the CPP test infrastructure:
+
+* Separate each translation unit into its own test executable
+* Have a unified script to control the execution of which set of tests
+:::
+
+## Adding a new test case
+
+Please follow [Googletest Primer](https://google.github.io/googletest/primer.html) and [Advanced googletest Topics](https://google.github.io/googletest/advanced.html).