build/tests: support declaring tests to be manually executed

backend.native/tests/build.gradle

@@ -1500,6 +1500,7 @@ task interop_echo_server(type: RunInteropKonanTest) {
     if (isLinux()) {
         disabled = true
     }
+    run = false
     source = "interop/basics/echo_server.kt"
     interop = 'sockets'
 }

backend.native/tests/interop/basics/echo_server.kt

@@ -2,7 +2,8 @@ import kotlinx.cinterop.*
 import sockets.*
 
 fun main(args: Array<String>) {
-    if (args.size == 0) {
+    if (args.size < 1) {
+        println("Usage: ./echo_server <port>")
         return
     }