Don't initialize logging.

Calling the logging initialization code causes roughly the following output to be printed:

687: Test command: /builds/gitlab-kitware-sciviz-ci/build/bin/UnitTests_vtkm_io_testing "UnitTestImageWriter" "--device=openmp" "-v" "INFO" "--data-dir=/builds/gitlab-kitware-sciviz-ci/data/data" "--baseline-dir=/builds/gitlab-kitware-sciviz-ci/data/baseline" "--write-dir=/builds/gitlab-kitware-sciviz-ci/build"
687: Test timeout computed to be: 180
687: date       time         ( uptime  ) [ thread name/id ]                   file:line     v| 
687: 2021-04-10 21:09:29.142 (   0.001s) [main thread     ]             loguru.cpp:591   Info| arguments: UnitTestImageWriter --device=openmp -v INFO --data-dir=/builds/gitlab-kitware-sciviz-ci/data/data --baseline-dir=/builds/gitlab-kitware-sciviz-ci/data/baseline --write-dir=/builds/gitlab-kitware-sciviz-ci/build
687: 2021-04-10 21:09:29.142 (   0.001s) [main thread     ]             loguru.cpp:594   Info| Current dir: /builds/gitlab-kitware-sciviz-ci/build/vtkm/io/testing
687: 2021-04-10 21:09:29.142 (   0.001s) [main thread     ]             loguru.cpp:596   Info| stderr verbosity: 0
687: 2021-04-10 21:09:29.142 (   0.001s) [main thread     ]             loguru.cpp:597   Info| -----------------------------------
687: 2021-04-10 21:09:29.165 (   0.024s) [main thread     ]             loguru.cpp:471   Info| atexit

The only relevant information (the test command) is exactly the same as what is printed by ctest.

Removing this superfluous information causes the size of the log to decrease by roughly 5,000 lines, depending on the build configuration.