docs: document sqlness-runner usage (#931)

docs: paste doc from greptime-doc

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

tests/README.md

@@ -0,0 +1,38 @@
+# Sqlness Test
+
+## Sqlness manual
+
+### Case file
+Sqlness has three types of file
+- `.sql`: test input, SQL only
+- `.result`: expected test output, SQL and its results
+- `.output`: different output, SQL and its results
+
+Both `.result` and `.output` are output (execution result) files. The difference is that `.result` is the
+the standard (expected) output, and `.output` is the error output. Therefore, if you see `.output` files generated,
+it means this test gets a different result and indicates it fails. You should
+check change logs to solve the problem.  
+
+You only need to write test SQL in `.sql` file, and run the test. On the first run it produces
+an `.output` file because there is no `.result` to compare with. If you can make sure the content in
+`.output` is correct, you can rename it to `.result`, which means it is the expected output.
+
+And at any time there should only be two file types, `.sql` and `.result` -- otherwise, an existing `.output`
+file means your test fails. That's why we should not ignore `.output` file type in `.gitignore`, instead, track
+it and make sure it doesn't exist.
+
+### Case organization
+The root dir of input cases is `tests/cases`. It contains several sub-directories stand for different test
+modes. E.g., `standalone/` contains all the tests to run under `greptimedb standalone start` mode.
+
+Under the first level of sub-directory (e.g. the `cases/standalone`), you can organize your cases as you like.
+Sqlness walks through every file recursively and runs them.
+
+## Run the test
+Unlike other tests, this harness is in a binary target form. You can run it with
+```shell
+cargo run --bin sqlness-runner
+```
+It automatically finishes the following procedures: compile `GreptimeDB`, start it, grab tests and feed it to
+the server, then collect and compare the results. You only need to check whether there are new `.output` files.
+If not, congratulations, the test is passed 🥳!