[MLIR] Fix Standalone dialect test to work in out-of-tree builds

When LLVM and MLIR are built as subprojects (via add_subdirectory),
the CMake configuration that indicates where the MLIR libraries are is
not necessarily in the same cmake/ directory as LLVM's configuration.
This patch removes that assumption about where MLIRConfig.cmake is
located.

(As an additional none, the %llvm_lib_dir substitution was never
defined, and so find_package(MLIR) in the build was succeeding for
other reasons.)

Reviewed By: stephenneuendorffer

Differential Revision: https://reviews.llvm.org/D103276