Build: add kotlin.build.useIRForLibraries to enable JVM IR for libraries separately

The next step for JVM IR adoption in our project is going to be enabling
`kotlin.build.useIR`, but keeping `kotlin.build.useIRForLibraries`
disabled until we fix all remaining ABI incompatibility issues for
library code.

libraries/kotlin.test/testng/build.gradle

@@ -41,6 +41,8 @@ compileTestKotlin {
     kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package"]
 }
 
+configureJvmIrBackend(project)
+
 if (includeJava9) {
     compileJava9Sources(project, 'kotlin.test.testng')
 }