SpringBoot Run tests in parallel when multiple workers are available

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

Comment From: 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.

Comment From: wilkinsona

With 2.4.x about to reach the end of its OSS support period, we should tackle this in 2.5.x and later.