Introduce TestGenerationContext

This commit polishes DefaultGenerationContext to make the method
that flushes generated classes more explicit. It now throws an
IOException and TestGenerationContext has been updated to handle
that to ease its use in code that can't throw such an exception.

As this use case is likely to happen outside the Spring Framework,
this commit adds such a convenience to spring-test as well.

Closes gh-28877

spring-core/src/test/java/org/springframework/aot/generate/DefaultGenerationContextTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.aot.generate;
 
+import java.io.IOException;
 import java.util.function.Consumer;
 
 import org.junit.jupiter.api.Test;
@@ -118,7 +119,7 @@ class DefaultGenerationContextTests {
 	}
 
 	@Test
-	void withNameKeepsTrackOfAllGeneratedFiles() {
+	void withNameKeepsTrackOfAllGeneratedFiles() throws IOException {
 		DefaultGenerationContext context = new DefaultGenerationContext(
 				new ClassNameGenerator(TestTarget.class), this.generatedFiles);
 		context.getGeneratedClasses().addForFeature("Test", typeSpecCustomizer);
@@ -132,7 +133,7 @@ class DefaultGenerationContextTests {
 	}
 
 	@Test
-	void withNameGeneratesUniqueName() {
+	void withNameGeneratesUniqueName() throws IOException {
 		DefaultGenerationContext context = new DefaultGenerationContext(
 				new ClassNameGenerator(Object.class), this.generatedFiles);
 		context.withName("Test").getGeneratedClasses()