Fix tests for coroutines setting in Gradle

ArgumentUtils#convertArgumentsToStringList does not
serialize an argument with default value,
so the string "-Xcoroutines=warn" was no longer present in logs.

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/CoroutinesIT.kt

@@ -112,7 +112,7 @@ class CoroutinesIT: BaseGradleIT() {
         }
 
         build("build") {
-            assertContains("-Xcoroutines=$coroutineSupport")
+            assertContains("args.coroutinesState=$coroutineSupport")
         }
     }
 }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -157,6 +157,8 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments>() : AbstractCo
             Coroutines.ERROR -> CommonCompilerArguments.ERROR
         }
 
+        logger.kotlinDebug { "args.coroutinesState=${args.coroutinesState}" }
+
         if (project.logger.isDebugEnabled) {
             args.verbose = true
         }