doc: note that tests/lean/run disables linters (#11595)

This PR documents that tests in `tests/lean/run/` run with
`-Dlinter.all=false`, and explains how to enable specific linters when
testing linter behavior.

🤖 Prepared with Claude Code

Author: kim-em

doc/dev/testing.md

@@ -51,6 +51,10 @@ All these tests are included by [src/shell/CMakeLists.txt](https://github.com/le
   codes and do not check the expected output even though output is
   produced, it is ignored.
 
+  **Note:** Tests in this directory run with `-Dlinter.all=false` to reduce noise.
+  If your test needs to verify linter behavior (e.g., deprecation warnings),
+  explicitly enable the relevant linter with `set_option linter.<name> true`.
+
 - [`tests/lean/interactive`](https://github.com/leanprover/lean4/tree/master/tests/lean/interactive/): are designed to test server requests at a
   given position in the input file. Each .lean file contains comments
   that indicate how to simulate a client request at that position.