Make dependencies on AssertJ and JUnit in `spring-core-test` optional

[sbrannen]

Overview

The spring-core-test artifact currently has required transitive dependencies on AssertJ and JUnit.

spring-framework/spring-core-test/spring-core-test.gradle

Lines 5 to 6 in 8ee09e5

	api("org.assertj:assertj-core")
	api("org.junit.jupiter:junit-jupiter-api")

In light of my findings in #34610 (comment), we should make the dependencies on AssertJ and JUnit optional like we do in spring-test.

spring-framework/spring-test/spring-test.gradle

Lines 34 to 43 in 8ee09e5

	optional("org.assertj:assertj-core")
	optional("org.hamcrest:hamcrest")
	optional("org.htmlunit:htmlunit") {
	exclude group: "commons-logging", module: "commons-logging"
	}
	optional("org.jetbrains.kotlin:kotlin-reflect")
	optional("org.jetbrains.kotlin:kotlin-stdlib")
	optional("org.jetbrains.kotlinx:kotlinx-coroutines-core")
	optional("org.jetbrains.kotlinx:kotlinx-coroutines-reactor")
	optional("org.junit.jupiter:junit-jupiter-api")

Related Issues

• Upgrade to spring-core-test 6.2.4 forces upgrade to JUnit 5.12 #34610