Reinstate the @⁠Inject Technology Compatibility Kit (TCK)

In commit 05ebca8677, the `public` modifier was removed from the
SpringAtInjectTckTests class, which prevents it from being run as a
JUnit 3 test class.

To address that, this commit adds the missing `public` modifier as well
as a a code comment to help prevent this from happening again.

In addition, this commit updates spring-context.gradle to ensure that
the JUnit Vintage test engine is always applied. However, that Gradle
configuration is unfortunately ignored due to how our TestConventions
class has been implemented. Thus, that issue will have to be addressed
separately.

Closes gh-34800

(cherry picked from commit e384389790)

spring-context/spring-context.gradle

@@ -58,3 +58,10 @@ dependencies {
 	testRuntimeOnly("org.javamoney:moneta")
 	testRuntimeOnly("org.junit.vintage:junit-vintage-engine") // for @Inject TCK
 }
+
+test {
+	description = "Runs JUnit Jupiter tests and the @Inject TCK via JUnit Vintage."
+	useJUnitPlatform {
+		includeEngines "junit-jupiter", "junit-vintage"
+	}
+}