Convert environment used by SpringBootTestContextLoader

This commit aligns `SpringBootTest`s to also use `ApplicationEnvironment`
instead of `StandardEnvironment`. This prevents the side-effect of active
profiles from `@ActiveProfiles` from being added to the environment when
doGetActiveProfiles is called. In this case, calling `addActiveProfiles()`
in the environment post processor would result in `@ActiveProfiles` being
added to the environment first, resulting in the wrong order.

The additional call to `setActiveProfiles()` is also not necessary when using
ApplicationEnvironment because that call was put in place to prevent the side-effect
which `ApplicationEnvironment` does not have.

Fixes gh-28530

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

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2012-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.test.context;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.env.ConfigurableEnvironment;
+import org.springframework.core.env.Environment;
+import org.springframework.mock.env.MockEnvironment;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.ContextConfiguration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests for {@link SpringBootTest @SpringBootTest} with a custom
+ * {@link Environment}.
+ *
+ * @author Madhura Bhave
+ */
+@SpringBootTest
+@ActiveProfiles({ "test1", "test2" })
+@ContextConfiguration(loader = SpringBootTestWithCustomEnvironmentTests.Loader.class)
+class SpringBootTestWithCustomEnvironmentTests {
+
+	@Autowired
+	private Environment environment;
+
+	@Test
+	void getActiveProfiles() {
+		assertThat(this.environment).isInstanceOf(MockEnvironment.class);
+		assertThat(this.environment.getActiveProfiles()).containsOnly("test1", "test2");
+	}
+
+	@Configuration
+	static class Config {
+
+	}
+
+	static class Loader extends SpringBootContextLoader {
+
+		@Override
+		protected ConfigurableEnvironment getEnvironment() {
+			return new MockEnvironment();
+		}
+
+	}
+
+}