Add prefix for BUILD_TESTING cmake option
Add prefix for BUILD_TESTING cmake option

This is helpful when using spdlog as a dependency (git submodule) when a
master project is not interested in spdlog tests. Using
"BUILD_TESTING" name may create a confusion.
Extra: BUILD_EXAMPLE variable already have a prefix.

COMBINE-lab/spdlog,icylord/spdlog,hunter-packages/spdlog,godbyk/spdlog,mihadyuk/spdlog,godbyk/spdlog,hunter-packages/spdlog,COMBINE-lab/spdlog,icylord/spdlog,mihadyuk/spdlog,mihadyuk/spdlog,godbyk/spdlog,COMBINE-lab/spdlog,hunter-packages/spdlog,icylord/spdlog