LIT creates "Output" folders for each directory with LIT tests.
Before this change, LIT tests were executed in source directory and
"Output" folders had to be removed manually. Git also showed them as
folders to be potentially tracked. This PR changes LIT execution to
build directory to avoid this problem. Similar approach is used in MLIR.

* [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

* [MLIR] Add config files for LIT

LIT is the LLVM testing tool that is used in LLVM and MLIR. It's
necessary to enable all the MLIR testing in nGraph. This will provide
a complementary finer grain level of testing than the one currently done
with gtest. LIT will allow testing each MLIR-based nGraph transformation
without having to run the whole nGraph pipeline. For example, we will be
able to write an nGraph dialect input, invoke an single MLIR transformation
on it and check that the output is the expected.

This PR adds the basic configuration files to enable LIT tool
in nGraph for MLIR Compiler. `affine_lowering/lit_test.mlir` is a
dummy test to exercise the new testing feature. Lit testing will be
enabled in CMAKE in a subsequent PR.

* Address feedback