Add XLinear mmap unittest

[weiliw-amz]

Issue #, if available:
N/A

Description of changes:
Add unit test for memory-mapped XLinear model compile, load & predict.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[OctoberChang]

@weiliw-amz , can we also add some test cases on the get attribute functions, to increase the coverage of pytest?

For example,

assert py_model.depth == mmap_model.depth
assert py_model.nr_features == mmap_model.nr_features
assert py_model.nr_labels == mmap_model.nr_labels
assert py_model.nr_codes == mmap_model.nr_codes

[weiliw-amz]

@weiliw-amz , can we also add some test cases on the get attribute functions, to increase the coverage of pytest?

For example,

assert py_model.depth == mmap_model.depth
assert py_model.nr_features == mmap_model.nr_features
assert py_model.nr_labels == mmap_model.nr_labels
assert py_model.nr_codes == mmap_model.nr_codes

Sounds good, added.