Thanks to a free Arcmutate OSS license, GitHub Actions now runs Pitest against
files changed in the context of a PR.
While there, update Pitest's configuration to ignore Refaster rule collection
classes, as mutations of those classes will not impact the associated unit
tests.
See:
- https://www.arcmutate.com
- https://pitest.org
As the former term references a class containing one or more `@BeforeTemplate`
methods, one or more `@Placeholder` methods and an optional `@AfterTemplate`,
while the latter term more narrowly references a single `@BeforeTemplate` or
`@AfterTemplate` method.
