Revise TestConventions to retain existing JUnit Platform options

This commit revises the implementation of TestConventions so that
existing JUnit Platform options from a pre-configured `test` task are
copied instead of overridden.

Closes gh-34827

spring-test/spring-test.gradle

@@ -105,14 +105,10 @@ test {
 	description = "Runs JUnit 4, JUnit Jupiter, and TestNG tests."
 	useJUnitPlatform {
 		includeEngines "junit-vintage", "junit-jupiter", "testng"
-		excludeTags "failing-test-case"
 	}
-	// We use `include` instead of `filter.includeTestsMatching`, since
-	// the latter results in some tests being executed/reported
-	// multiple times.
-	include(["**/*Tests.class", "**/*Test.class"])
+	// `include` test filters and system properties are configured in
+	// org.springframework.build.TestConventions in buildSrc.
 	filter.excludeTestsMatching("*TestCase")
-	systemProperty("testGroups", project.properties.get("testGroups"))
-	// Java Util Logging for the JUnit Platform.
+	// Optionally configure Java Util Logging for the JUnit Platform.
 	// systemProperty("java.util.logging.manager", "org.apache.logging.log4j.jul.LogManager")
 }