Add failing pickling test to blacklist

tuples1.scala is explainable and harmless. The other two (i4758 and t7426) give a stale
symbol but in my setup they do that already on master. I wonder how they slipped through the CI.

Author: odersky

compiler/test/dotc/pos-test-pickling.blacklist

@@ -33,6 +33,7 @@ i4125.scala
 i4167
 i4586.scala
 i4734
+i4758
 implicit-dep.scala
 inline-access-levels
 macro-with-array
@@ -65,6 +66,7 @@ t5504
 t6028
 t6976
 t7264
+t7426.scala
 t7532b
 t8062
 typeclass-encoding2.scala

compiler/test/dotc/run-test-pickling.blacklist

@@ -75,6 +75,7 @@ transparentAccess
 transparentByName.scala
 transparentForeach.scala
 transparentPower
+tuples1.scala
 Tuple.scala
 TupleAbstract.scala
 typelevel-numeric.scala