Tidy up test migration and fix Gradle 9 test compatibility

build.gradle

@@ -66,7 +66,6 @@ dependencies {
     implementation 'com.palantir.gradle.idea-configuration:gradle-idea-configuration'
 
     testImplementation platform('org.junit:junit-bom')
-    testImplementation 'com.netflix.nebula:nebula-test'
     testImplementation 'org.mockito:mockito-core'
     testImplementation 'com.fasterxml.jackson.core:jackson-databind'
     testImplementation "org.junit.jupiter:junit-jupiter-api"
@@ -79,6 +78,8 @@ dependencies {
 
     testAnnotationProcessor "org.immutables:value"
     testCompileOnly "org.immutables:value::annotations"
+
+    gradlePluginForTesting 'com.palantir.sls-packaging:gradle-sls-packaging'
 }
 
 tasks.withType(Test) {

gradle/gradle-test-versions.yml

@@ -1,3 +1,5 @@
 major-versions:
+  7: 7.6.5
   8: 8.14.3
+  9: 9.4.0
 extra-versions: []