fix #16256: nimout: <empty> should give error (vacuously true); improve a few tests (#18089)

* fix #16256: nimout: <empty> should give error (vacuously true); improve some tests

* renamed:    tests/stdlib/t9710.nim -> tests/misc/t9710.nim

* improve tests

* fix non-DRY tests

* improve $nim_prs_D/tests/stdlib/t9091.nim

* renamed:    tests/stdlib/t9091.nim -> tests/misc/t9091.nim

* fixup

* address comment: doAssert => result.parseErrors

tests/stdlib/t9091.nim

@@ -1,36 +0,0 @@
-discard """
-  targets:  "c"
-  output:   "test AObj"
-  action:   "compile"
-  exitcode: 0
-  timeout:  60.0
-"""
-import streams
-
-block:
-  type Mine = ref object
-    a: int
-
-  proc write(io: Stream, t: Mine) =
-    io.write("sure")
-
-  let str = newStringStream()
-  let mi = new Mine
-
-  str.write(mi)
-
-block:
-  type
-    AObj = object
-      x: int
-
-  proc foo(a: int): string = ""
-
-  proc test(args: varargs[string, foo]) =
-    echo "varargs"
-
-  proc test(a: AObj) =
-    echo "test AObj"
-
-  let x = AObj()
-  test(x)