Use consistent package name for test modules

It’s a really trivial nit-pick, but it seems we’ve been pretty inconsistent with our test package names in the past:

$ grep -h ^package: -- tests/*/*.info.yml | sort | uniq -c
      5 package: Search
      3 package: 'Search API'
      3 package: Testing

As Core uses “Testing” very consistently (which is really amazing, it being Core), we should switch to that as well, I’d say.