Due to `@ConditionalOnClass` and `@ConditionalOnMissingClass`, the
behaviour of many auto-configuration classes is dependant on the
contents of the class path, yet we do not have a lightweight way of
testing such classes against a specific class path.

This commit introduces FilteredClassPathRunner, a JUnit Runner that
runs a class’s tests using a filtered class path. A
`@ClassPathExclusions` annotation on a test class can be used to
filter entries from the project’s default class path, thereby allowing
a configuration class’s behaviour in the presence or absence of
certain classes to be tested more easily.

Closes gh-5359