NNXSW-1826 Move tests for Optimization classes to separate files

NNXSW-1826 Move tests for Optimization classes to separate files

This splits up the >1000 line OptimizerTests.cpp file.

Each Optimization class now has its own test file, all of which are in a
subfolder of tests called "optimizations".

The original OptimizerTests.cpp now contains mostly (completely?) tests
for validating output shapes, which perhaps should be moved to
test files specific to the layer types they are testing.

Change-Id: Icd1196cad8b720abcb156921aab1adbd4026756b
Signed-off-by: Rob Hughes <robert.hughes@arm.com>