Move unittests related to manila/share/*.py modules to proper place

It is expected, that unittests cover project modules in relation 1:1 and
placed in appropriate tree as main modules do.

It should be like in example:
manila/[tests/]foo/bar/[test_]modulename.py

Existing file 'manila/tests/test_share.py' will be merged with existing file
'manila/tests/share/test_manager.py' separately.

Change-Id: Ifd9dc13f4632b255dae81b521a05d47838b3042c
Partial-Fix: #1348633