Fix warnings from GCC 7.3.0

[taketwo]

This fixes a few warnings from GCC 7.3.0 in test sources.

../test/common/test_eigen.cpp:748:12: warning: unused variable ‘c_fail_count’ [-Wunused-variable]
../test/octree/test_octree.cpp:342:10: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wdangling-else]
../test/segmentation/test_random_walker.cpp:232:10: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wdangling-else]

[SunBlack]

Maybe you could add this to CMake to prevent new issues like this:

if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang" OR CMAKE_COMPILER_IS_GNUCXX)
	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wunused-variable")
endif()

[taketwo]

I actually got these without setting any extra flags. We'll enable Ubuntu 18 build on CI soon and will catch these.

[SunBlack]

Sorry, copy & paste error.

I meant: -Werror=unused-variable instead of just -Wunused-variable.