Fix return type of getIndexable in HappyTreeFriends

[aprokop]

First push is the test to make sure CI gets the warning.
Second push is the fix.

[aprokop]

Warning from GCC build on 0d18194:

In file included from <snip>/test/tstIndexableGetter.cpp:14:
In file included from <snip>/src/ArborX_LinearBVH.hpp:27:
In file included from <snip>/src/details/ArborX_DetailsTreeTraversal.hpp:15:
<snip>/src/details/ArborX_DetailsHappyTreeFriends.hpp:56:12:
warning: returning reference to local temporary object [-Wreturn-stack-address]
    return bvh._indexable_getter(getValue(bvh, i));
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~