Clean up Octree unit tests

[taketwo]

(follow-up for #1532)

• Use appropriate GTest assertions
• Enforce PCL style
• Delete trailing spaces

[SergioRAgostinho]

Thanks. 👍

Just for reference, from https://github.com/google/googletest/blob/master/googletest/docs/Primer.md

Historical note: Before February 2016 *_EQ had a convention of calling it as ASSERT_EQ(expected, actual), so lots of existing code uses this order. Now *_EQ treats both parameters in the same way.

Although I'm still in favour of retaining the same order convention for a while.

[taketwo]

Thanks, did not know this. But yeah, my GTest on Ubuntu 16.04 still prints in case of failure something like:

/home/sergey/Workspace/Libraries/pcl/test/octree/test_octree.cpp:1669: Failure
Value of: leaf_container->size ()
  Actual: 18
Expected: 27

so better have it in the conventional right order for the time being.