Support custom SpringBootContextLoader environment

Provide a `getEnvironment` method in `SpringBootContextLoader` to allow
specialized `ConfigurableEnvironment` implementations to be used.

Closes gh-10740

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootContextLoader.java

@@ -105,7 +105,7 @@ public class SpringBootContextLoader extends AbstractContextLoader {
 		application.setMainApplicationClass(config.getTestClass());
 		application.addPrimarySources(Arrays.asList(configClasses));
 		application.getSources().addAll(Arrays.asList(configLocations));
-		ConfigurableEnvironment environment = new StandardEnvironment();
+		ConfigurableEnvironment environment = getEnvironment();
 		if (!ObjectUtils.isEmpty(config.getActiveProfiles())) {
 			setActiveProfiles(environment, config.getActiveProfiles());
 		}
@@ -148,6 +148,15 @@ public class SpringBootContextLoader extends AbstractContextLoader {
 		return new SpringApplication();
 	}
 
+	/**
+	 * Builds a new {@link ConfigurableEnvironment} instance. You can override this method
+	 * to return something other than {@link StandardEnvironment} if necessary.
+	 * @return a {@link ConfigurableEnvironment} instance
+	 */
+	protected ConfigurableEnvironment getEnvironment() {
+		return new StandardEnvironment();
+	}
+
 	private void setActiveProfiles(ConfigurableEnvironment environment,
 			String[] profiles) {
 		TestPropertyValues