Ensure that all guide 'main' functions return Unit

Author: elizarov

kotlinx-coroutines-core/jvm/test/guide/test/TestUtil.kt

@@ -26,7 +26,7 @@ private const val SHUTDOWN_TIMEOUT = 5000L // 5 sec at most to wait
 private val OUT_ENABLED = systemProp("guide.tests.sout", false)
 
 @Suppress("DEPRECATION")
-fun test(name: String, block: () -> Unit): List<String> = outputException(name) {
+fun <R> test(name: String, block: () -> R): List<String> = outputException(name) {
     val sout = System.out
     val oldOut = if (OUT_ENABLED) System.out else NullOut
     val oldErr = System.err
@@ -45,7 +45,8 @@ fun test(name: String, block: () -> Unit): List<String> = outputException(name)
     var bytes = ByteArray(0)
     withVirtualTimeSource(oldOut) {
         try {
-            block()
+            val result = block()
+            require(result === Unit) { "Test 'main' shall return Unit" }
         } catch (e: Throwable) {
             System.err.print("Exception in thread \"main\" ")
             e.printStackTrace()