[mlir][test] Link MLIRBufferizationTestPasses to MLIRTestDialect directly (#238752)
#236155 removed the explicit test dialect include directories from
MLIRBufferizationTestPasses, relying on the usage requirements of
MLIRTestDialect instead. The library linked MLIRTestDialect via
`mlir_target_link_libraries`, which links libMLIR instead of the given
libraries if `MLIR_LINK_MLIR_DYLIB` is enabled. Since the test dialect
is not part of libMLIR, it was never linked, and its include directories
were never added, causing `TestDialect.h` to not be found.
Link MLIRTestDialect via `LINK_LIBS` like all other test libraries.