Avoid deadlock in runtest t5375.scala

See #624 (comment)
for a lengthy explanation.

Author: smarter

tests/run/t5375.scala

@@ -1,8 +1,11 @@
-object Test extends dotty.runtime.LegacyApp {
+object Test {
   val foos = (1 to 1000).toSeq
-  try
-    foos.par.map(i => if (i % 37 == 0) sys.error("i div 37") else i)
-  catch {
-    case ex: RuntimeException => println("Runtime exception")
+
+  def main(args: Array[String]): Unit = {
+    try
+      foos.par.map(i => if (i % 37 == 0) sys.error("i div 37") else i)
+    catch {
+      case ex: RuntimeException => println("Runtime exception")
+    }
   }
 }