Fix Sorbet version used in tests

[egiurleo]

Problem
The spoom Gemfile does not lock the version of Sorbet, so by default bundler installs the latest version of Sorbet on every test run. This can cause test failures that are unrelated to a given PR (see this example), which is confusing and blocks us from merging PRs without breaking changes.

Solution
Let's fix the Sorbet version used in tests to the one listed in Gemfile.lock (here's how we do it in Tapioca). Any breaking change in Sorbet will only cause the tests to fail when Sorbet is bumped, which is much easier to track and fix than with our current approach.