Do not blank opt in for allowing kotlin package in all subprojects

-Xallow-kotlin-package must be specified explicitly to prevent
unintended declarations in kotlin package.

libraries/kotlin.test/common/build.gradle

@@ -31,12 +31,16 @@ artifacts {
 
 javadocJar()
 
+tasks.withType(org.jetbrains.kotlin.gradle.dsl.KotlinCompile) {
+    kotlinOptions.freeCompilerArgs += "-Xallow-kotlin-package"
+}
+
 compileKotlinCommon {
     kotlinOptions {
-        freeCompilerArgs = [
+        freeCompilerArgs += [
                 "-module-name", project.name,
-                "-Xallow-kotlin-package",
                 "-Xuse-experimental=kotlin.contracts.ExperimentalContracts"
         ]
     }
-}
+}
+