Modify SpringApplication Environment rather than setting it

Update `SpringBootContextLoader` so that when possible the
`SpringApplication` remains in control of creating the `Environment`
instance.

Prior to this commit, we would always create the `Environment` in the
`SpringBootContextLoader` and then call `setEnvironment` on the
`SpringApplication`. This meant that the `ApplicationEnvironment`
classes were not used and that `isCustomEnvironment` was set to `true`
so no conversion was applied.

With the updated code, an `ApplicationListener` is used to mutate the
`Environment` instance and add the required test property sources.

Fixes gh-29169

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-profile/src/test/java/smoketest/profile/ActiveProfilesTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.env.EnvironmentPostProcessor;
 import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
 import org.springframework.core.env.Environment;
 import org.springframework.test.context.ActiveProfiles;
 
@@ -32,7 +33,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Madhura Bhave
  */
-@SpringBootTest(properties = "enableEnvironmentPostProcessor=true") // gh-28530
+@SpringBootTest(webEnvironment = WebEnvironment.NONE, properties = "enableEnvironmentPostProcessor=true") // gh-28530
 @ActiveProfiles("hello")
 class ActiveProfilesTests {