Restore forgotten fatal-warnings tests

In daeee35, support for tests in the directory
`tests/pos-special/fatal-warnings` was dropped in favor of magic `//>` comments
in the regular `tests/pos` directory, but a few tests were forgotten in the
original directory and were thus not run. This commit moves them to the
appropriate directory with the correct magic comment.

[Cherry-picked 0ee804f][modified]

Author: WojciechMazur

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

@@ -28,6 +28,7 @@ i13433.scala
 i16649-irrefutable.scala
 strict-pattern-bindings-3.0-migration.scala
 i17255
+i17735.scala
 
 # Tree is huge and blows stack for printing Text
 i7034.scala

tests/pos-special/fatal-warnings/i17735.scala renamed to tests/pos/i17735.scala

@@ -1,4 +1,4 @@
-// scalac: -Wvalue-discard
+//> using options -Xfatal-warnings -Wvalue-discard
 
 import scala.collection.mutable
 import scala.annotation.nowarn
@@ -21,4 +21,4 @@ object Foo:
       // here @nowarn is effective without -Wfatal-warnings (i.e. no warning)
       // But with -Wfatal-warnings we get an error
       messageBuilder.append("\n").append(s): @nowarn("msg=discarded non-Unit value*")
-    messageBuilder.result()
+    messageBuilder.result()

tests/pos-special/fatal-warnings/i17735a.scala renamed to tests/pos/i17735a.scala

@@ -1,4 +1,4 @@
-// scalac: -Wvalue-discard -Wconf:msg=non-Unit:s
+//> using options -Xfatal-warnings -Wvalue-discard -Wconf:msg=non-Unit:s
 
 import scala.collection.mutable
 import scala.annotation.nowarn

tests/pos-special/fatal-warnings/i17741.scala renamed to tests/pos/i17741.scala

@@ -1,4 +1,4 @@
-// scalac: -Wnonunit-statement
+//> using options -Xfatal-warnings -Wnonunit-statement
 
 class Node()
 class Elem(
@@ -29,4 +29,4 @@ object Main {
       )
     }
   }: @annotation.nowarn()
-}
+}

tests/pos-special/fatal-warnings/nowarnannot.scala renamed to tests/pos/nowarnannot.scala

@@ -1,3 +1,5 @@
+//> using options -Xfatal-warnings -Wvalue-discard
+
 case class F(i: Int)
 
 object Main {