refactor: only include lib in $LOAD_PATH if not included yet

[pvdb]

When running the rake tests using bundle exec rake test, the lib directory is added to $LOAD_PATH twice, once by the Rakefile, and again by test_helper.rb ... the change in this PR only adds it if it isn't present already, following the idiom that's already used by the Rakefile:

lib = File.expand_path("../lib", __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)

Consistency FTW ... thanks @hsbt ! 🙏