[MLIR] Update tutorial to add missing tests and bring directory paths and code snippe...

[MLIR] Update tutorial to add missing tests and bring directory paths and code snippets up to date.

Summary:
The tests referred to in Chapter 3 of the tutorial were missing from the tutorial test
directory; this adds those missing tests. This also cleans up some stale directory paths and code
snippets used throughout the tutorial.

Differential Revision: https://reviews.llvm.org/D76809