Use ChefSpec::SoloRunner, not ServerRunner

SoloRunner appears to be the preferred test runner these days, and is faster than ServerRunner. Many of the examples on https://github.com/chefspec/chefspec have been converted to SoloRunner.

"Appears" is a bit of a weasel word but it's frustrating to find concrete answers on this topic. Basically, I found talking heads on stackoverflow and discourse claiming ServerRunner is deprecated, the official docs have broadly switched to using it in examples, and switching sped up my ChefSpec runs on a new cookbook I'm developing. I switched gitlab_fluentd over (without committing it yet) and the runtime dropped from 115s to 16s.