* [MLIR] Enable LIT testing in CMAKE

This PR enables LIT testing in nGraph for MLIR Compiler, introduced in
PR3523. We can now do `make check-mlir-lit` to run LIT tests.

* Address feedback

* Revisit PR after cmake clean-up

* [MLIR] Introduce ngraph-opt tool

To be used, among other things, for LIT testing.
`elementwise_binary_ops.mlir` shows a simple LIT test
using ngraph-opt.

Dummy affine_lowering/lit_test.mlir test that served as initial testing
of LIT tool configuration is now removed.

* Address feedback