Nopos-warn in vulpix & Move warn tests form tests/neg to tests/warn: Batch 1 (#19242)

Adds functionality to vulpix that allows putting nopos-warns in comments
& First batch of tests moved from tests/neg to tests/warn.
PR 2/5 (merge consecutively, per [Nicolas'
suggestion](#18829 (review))

Split up version of #18829

Author: szymon-rd

tests/neg-deep-subtype/i3324.scala

@@ -0,0 +1,6 @@
+-- Error: tests/neg/i11225b.scala:6:26 ---------------------------------------------------------------------------------
+6 |  private var cached: A = _ // error
+  |                          ^
+  |                          `= _` has been deprecated; use `= uninitialized` instead.
+  |                          `uninitialized` can be imported with `scala.compiletime.uninitialized`.
+  |                          This construct can be rewritten automatically under -rewrite -source 3.4-migration.

tests/neg/i10247.scala

@@ -1,4 +1,4 @@
-//> using options -source future -deprecation -Xfatal-warnings
+//> using options -source future -deprecation
 
 import compiletime.uninitialized
 
@@ -10,3 +10,4 @@ class Memo[A](x: => A):
       known = true
       cached = x
     cached
+