Fix directory layout for tools/memory_inspector tests.

This cl rearranges the directory layout for the memory_inspector tests.
This is to ensure that:
 - Every unittest lives side-by-side with the module being tested.
 - The mocks/helper class live in the unittest package.
 - Nobody has to hack the sys.path ever.
 - Every (test and non) module uses the "import memory_inspector.submodule"
   pattern consistently, without creating any namespace ambiguities.

BUG=340294

Review URL: https://codereview.chromium.org/167223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251413 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251413