[llvm] [RISCV][llvm-exegesis] Add unittests. NFC (PR #121862)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vla` running on `linaro-g3-01` while building `llvm` at step 6 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/17/builds/5013

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (build stage 1) failure: 'ninja' (failure)
...
[7905/8797] Building CXX object tools/clang/tools/libclang/CMakeFiles/libclang.dir/CIndex.cpp.o
[7906/8797] Building CXX object tools/flang/lib/Optimizer/Builder/CMakeFiles/FIRBuilder.dir/CUFCommon.cpp.o
[7907/8797] Building CXX object tools/flang/lib/Optimizer/Builder/CMakeFiles/FIRBuilder.dir/FIRBuilder.cpp.o
[7908/8797] Building CXX object tools/flang/lib/Optimizer/Builder/CMakeFiles/FIRBuilder.dir/HLFIRTools.cpp.o
[7909/8797] Building CXX object tools/clang/tools/libclang/CMakeFiles/libclang.dir/CXExtractAPI.cpp.o
[7910/8797] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/ComponentPath.cpp.o
[7911/8797] Building CXX object tools/flang/lib/Lower/CMakeFiles/FortranLower.dir/HlfirIntrinsics.cpp.o
[7912/8797] Building CXX object tools/flang/lib/Optimizer/Builder/CMakeFiles/FIRBuilder.dir/PPCIntrinsicCall.cpp.o
[7913/8797] Linking CXX static library lib/libclangDaemonTweaks.a
[7914/8797] Building CXX object tools/flang/lib/Optimizer/Builder/CMakeFiles/FIRBuilder.dir/IntrinsicCall.cpp.o
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2232.233302

```

</details>

https://github.com/llvm/llvm-project/pull/121862