Use tags for Spectrum's own test suite

[greghaskins]

Some of Spectrum's tests are more integration-focused and slower to run (e.g. those for Spring, etc.). It would be cool to use the tagging feature to split those out so they can get run on CI without slowing down the local TDD cycle. It's not really that slow yet, but down the line I see the possibility of integrating more static analysis (such as degraph) and other types of tests, which would make sense to keep split from the unit test suite. Plus, dogfooding is good.