[mlir][sparse] Add Merger unit tests (with gcc5 build fix)

This is a fix of https://reviews.llvm.org/D104956, which broke the gcc5 build.

We opt to use unit tests rather than check tests as the lattice/merger code is a small C++ component with a well-defined API. Testing this API via check tests would be far less direct and readable. In addition, as the check tests will only be able to test the API indirectly, the tests may break based on unrelated changes; e.g. changes in linalg.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D105828