Update BootRun to support Gradle's configuration cache

See gh-22922

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootWarIntegrationTests-applicationPluginMainClassNameIsUsed.gradle

@@ -4,4 +4,4 @@ plugins {
 	id 'org.springframework.boot' version '{version}'
 }
 
-mainClass = 'com.example.CustomMain'
+mainClassName = 'com.example.CustomMain'

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/run/BootRunIntegrationTests-applicationPluginMainClassNameIsNotUsedWhenItIsNull.gradle

@@ -3,9 +3,8 @@ plugins {
 	id 'org.springframework.boot' version '{version}'
 }
 
-task echoMainClassName {
-	dependsOn compileJava
-	doLast {
-		println 'Main class name = ' + bootRun.main
+bootRun {
+	doFirst {
+		println "Main class name = ${bootRun.main}"
 	}
-}
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/run/BootRunIntegrationTests-applicationPluginMainClassNameIsUsed.gradle

@@ -3,8 +3,4 @@ plugins {
 	id 'org.springframework.boot' version '{version}'
 }
 
-mainClassName = 'com.example.CustomMainClass'
-
-task echoMainClassName {
-	println 'Main class name = ' + bootRun.main
-}
+mainClassName = 'com.example.main.CustomMainClass'

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/run/BootRunIntegrationTests-springBootExtensionMainClassNameIsUsed.gradle

@@ -1,12 +1,8 @@
 plugins {
-	id 'application'
+	id 'java'
 	id 'org.springframework.boot' version '{version}'
 }
 
 springBoot {
-	mainClass = 'com.example.CustomMainClass'
-}
-
-task echoMainClassName {
-	println 'Main class name = ' + bootRun.main
+	mainClass = 'com.example.main.CustomMainClass'
 }