[llvm] [BOLT][Docs] Add instructions to run all tests. (PR #172847)

[Gergely Bálint via llvm-commits]

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@@ -59,6 +59,34 @@ Start with cloning LLVM repo:
 `llvm-bolt` will be available under `bin/`. Add this directory to your path to
 ensure the rest of the commands in this tutorial work.
 
+### Testing changes during development
+
+During development, it is recommended to run both the in-tree tests (under
+[bolt/test](./test)) and the out-of-tree tests, which use binaries hosted in an
+[external](https://github.com/rafaelauler/bolt-tests) repository.
+
+Below are sample instructions to run both test suites. You may need to define
+additional CMake variables.
+
+```bash
+git clone https://github.com/llvm/llvm-project.git
+git clone https://github.com/rafaelauler/bolt-tests
+mkdir build
+cmake -G Ninja -S llvm-project/llvm -B build \
+       -DLLVM_TARGETS_TO_BUILD='AArch64;X86' \
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bgergely0 wrote:

nit: can you add RISCV as well, to have all 3 supported architectures?

https://github.com/llvm/llvm-project/pull/172847