Support partial test case running

[sj26]

We use rspec tags to run our test suite in pieces. Some of these pieces are based on test type, like system tests which require a heavier setup phase, and can also be distinguishes by file path. But we also have tests throughout out suite which require features only available in local development environments, e.g. a fully functioning privileged docker environment, which are throughout the test suite (not distinguishable by path). We use rspec tags to filter these out (rspec --tag ~docker or SPEC_OPTS="--tag ~docker"). But test splitter doesn't seem to understand these, so splits across files which have no examples, and this can lead to us running jobs that run no tests at all (because they all get filtered out).

e.g.

The SplitByExample function looks like it would do what we need, but only operates on slow test files. Is there a way to always split by example, or do smarter discovery of the test suite?