Add missing JUnit launcher

For some reason, this was missing only from the hangman and rest-api exercises.
It is required to run the tests from Gradle.

See https://docs.gradle.org/current/userguide/upgrading_version_8.html#test_framework_implementation_dependencies

Author: kahgoh

exercises/practice/hangman/build.gradle

@@ -12,6 +12,8 @@ dependencies {
     testImplementation platform("org.junit:junit-bom:5.10.0")
     testImplementation "org.junit.jupiter:junit-jupiter"
     testImplementation "org.assertj:assertj-core:3.25.1"
+
+    testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
 }
 
 test {

exercises/practice/rest-api/build.gradle

@@ -12,6 +12,8 @@ dependencies {
     testImplementation platform("org.junit:junit-bom:5.10.0")
     testImplementation "org.junit.jupiter:junit-jupiter"
     testImplementation "org.assertj:assertj-core:3.25.1"
+
+    testRuntimeOnly "org.junit.platform:junit-platform-launcher"
 }
 
 test {