Apply 'EXCLUDE_FROM_ALL' to the directory when ONNX_MLIR_BUILD_TESTS is not ON

[mschofie]

Fixes #3285.

ONNX_MLIR_BUILD_TESTS is documented as controlling whether the test executables are built. But it doesn't appear to be honored - even when I set it to OFF, the tests are still built. The problem appears to be with the use of EXCLUDE_FROM_ALL - when attempting to opt-out of tests, the CMake variable EXCLUDE_FROM_ALL is set to ON - however CMake only documents EXCLUDE_FROM_ALL properties applied to Directories (documentation) or Targets (documentation). This change:

1. Updates the logic that tests ONNX_MLIR_BUILD_TESTS to set the EXCLUDE_FROM_ALL property on the current Directory.
2. Moves the check from :/test/multiple-models/CMakeLists.txt up to :/test/CMakeLists.txt to apply to all of :/test

I've validated with "Ninja Multi-Config" and "Visual Studio 17 2022" generators on Windows. Without the fix, running a CMake build with no target specified, and ONNX_MLIR_BUILD_TESTS set to OFF will build the test targets. With the fix, the test targets aren't built by default, but will build when a target is specifically requested.

[jenkins-droid]

Can one of the admins verify this patch?