Run tests in parallel when multiple workers are available

[wilkinsona]

The test task of spring-boot-autoconfigure is often the long-tail of the build. We should configure it to use multiple workers.

[wilkinsona]

Have some more work to do to the build before it's ready for this. For example, RestAssuredRestDocsAutoConfigurationIntegrationTests, WebTestClientRestDocsAutoConfigurationIntegrationTests, and MockMvcRestDocsAutoConfigurationIntegrationTests all write to and recursively delete build/generated-snippets. This can lead to test failures when they end up running in parallel.