Increase test timeout

This should prevent false-negatives on unpredictably slow veficiation
tests such as invalid/RedBlackTree.

src/test/scala/leon/test/LeonTestSuite.scala

@@ -102,7 +102,7 @@ trait LeonTestSuite extends FunSuite with Timeouts {
 
       testContext = generateContext
 
-      failAfter(2.minutes) {
+      failAfter(5.minutes) {
         body
       }