Introduce TestGenerationContext

[snicoll]

Looking at #28684, it became apparent that Closeable wasn't really an option but that we could make writeGeneratedContent more prominent and let it throws an IOException. This had a knock-off effect on the tests which was easily handled using our internal TestGenerationContext.

This impacts obviously tests outside of the Spring Framework so a public variant of it is needed.

[sbrannen]

Reopening to move TestGenerationContext from spring-test to spring-core-test.

[snicoll]

Fixed by 58b0251 which also removes the test fixtures copy!