[mlir] Move Linalg tensors-to-buffers tests to Linalg tests.

The buffer placement preparation tests in
test/Transforms/buffer-placement-preparation* are using Linalg as a test
dialect which leads to confusion and "copy-pasta", i.e. Linalg is being
extended now and when TensorsToBuffers.cpp is changed, TestBufferPlacement is
sometimes kept in-sync, which should not be the case.

This has led to the unnoticed bug, because the tests were in a different directory and the patterns were slightly off.

Differential Revision: https://reviews.llvm.org/D89209