[llvm] [llvm][Docs] Make it clear where lit test files live (PR #124121)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-dylib` running on `linaro-flang-aarch64-dylib` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/50/builds/9342

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
534.066 [1538/10/5273] Building CXX object tools/mlir/lib/Dialect/NVGPU/Utils/CMakeFiles/obj.MLIRNVGPUUtils.dir/MMAUtils.cpp.o
534.067 [1538/9/5274] Building CXX object tools/mlir/lib/Dialect/PDLInterp/IR/CMakeFiles/obj.MLIRPDLInterpDialect.dir/PDLInterp.cpp.o
534.070 [1538/8/5275] Building CXX object tools/mlir/lib/Dialect/NVGPU/Transforms/CMakeFiles/obj.MLIRNVGPUTransforms.dir/CreateAsyncGroups.cpp.o
534.122 [1538/7/5276] Building CXX object tools/mlir/lib/Dialect/PDL/IR/CMakeFiles/obj.MLIRPDLDialect.dir/PDLTypes.cpp.o
534.125 [1538/6/5277] Building CXX object tools/mlir/lib/Dialect/MLProgram/Transforms/CMakeFiles/obj.MLIRMLProgramTransforms.dir/BufferizableOpInterfaceImpl.cpp.o
534.127 [1538/5/5278] Building CXX object tools/mlir/test/lib/Dialect/ArmSME/CMakeFiles/MLIRArmSMETestPasses.dir/TestLowerToArmSME.cpp.o
534.129 [1538/4/5279] Building CXX object tools/mlir/lib/Dialect/OpenMP/CMakeFiles/obj.MLIROpenMPDialect.dir/IR/OpenMPDialect.cpp.o
534.130 [1538/3/5280] Building CXX object tools/mlir/lib/Dialect/Polynomial/IR/CMakeFiles/obj.MLIRPolynomialDialect.dir/PolynomialAttributes.cpp.o
534.133 [1538/2/5281] Building CXX object tools/mlir/lib/Dialect/PDL/IR/CMakeFiles/obj.MLIRPDLDialect.dir/PDL.cpp.o
551.654 [1538/1/5282] Building CXX object tools/mlir/test/lib/IR/CMakeFiles/MLIRTestIR.dir/TestSymbolUses.cpp.o
FAILED: tools/mlir/test/lib/IR/CMakeFiles/MLIRTestIR.dir/TestSymbolUses.cpp.o 
/usr/local/bin/c++ -DGTEST_HAS_RTTI=0 -DMLIR_INCLUDE_TESTS -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/build/tools/mlir/test/lib/IR -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/mlir/test/lib/IR -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/build/tools/mlir/include -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/mlir/include -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/build/include -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/llvm/include -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/mlir/test/lib/IR/../Dialect/Test -I/home/tcwg-buildbot/worker/flang-aarch64-dylib/build/tools/mlir/test/lib/IR/../Dialect/Test -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -Wundef -Werror=mismatched-tags -O3 -DNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT tools/mlir/test/lib/IR/CMakeFiles/MLIRTestIR.dir/TestSymbolUses.cpp.o -MF tools/mlir/test/lib/IR/CMakeFiles/MLIRTestIR.dir/TestSymbolUses.cpp.o.d -o tools/mlir/test/lib/IR/CMakeFiles/MLIRTestIR.dir/TestSymbolUses.cpp.o -c /home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/mlir/test/lib/IR/TestSymbolUses.cpp
In file included from /home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/mlir/test/lib/IR/TestSymbolUses.cpp:9:
/home/tcwg-buildbot/worker/flang-aarch64-dylib/llvm-project/mlir/test/lib/IR/../Dialect/Test/TestOps.h:148:10: fatal error: 'TestOps.h.inc' file not found
  148 | #include "TestOps.h.inc"
      |          ^~~~~~~~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/124121