Make test t6827 less flaky

It failed once on the CI on OpenJDK 8 because the toString of the
exception was "java.lang.ArrayIndexOutOfBoundsException" instead of
"java.lang.ArrayIndexOutOfBoundsException:-1"
(https://dotty-ci.epfl.ch/lampepfl/dotty/982/1/4). I have no idea how
this could happen (different code paths between the interpreted and JIT
compiled code ?), so just stop checking for the exact toString output.

Author: smarter

tests/run/t6827.check

@@ -1,5 +1,5 @@
-start at -5: java.lang.ArrayIndexOutOfBoundsException: -5
-start at -1: java.lang.ArrayIndexOutOfBoundsException: -1
+start at -5: OOB
+start at -1: OOB
 start at limit: ok
 start at limit-1: ok
 first 10: ok

tests/run/t6827.scala

@@ -9,16 +9,9 @@ object Test extends App {
       "ok"
     } catch {
       case e: ArrayIndexOutOfBoundsException =>
-        // Special-case printing this exception because the toString changed in Java 11
-        val java11toString = """java.lang.ArrayIndexOutOfBoundsException: Index (-?\d+).*""".r
-
-        e.toString match {
-          case java11toString(index) =>
-            s"java.lang.ArrayIndexOutOfBoundsException: $index"
-          case str =>
-            str
-        }
-      case e: Exception => e.toString
+        "OOB"
+      case e: Exception =>
+        e.toString
     }
     println("%s: %s" format (label, status))
   }