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/misc/t9091.nim

@@ -0,0 +1,33 @@
+# bug #9091
+
+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)
+  str.setPosition 0
+  doAssert str.readAll == "sure"
+
+block:
+  type
+    AObj = object
+      x: int
+
+  proc foo(a: int): string = ""
+
+  proc test(args: varargs[string, foo]) =
+    doAssert false
+
+  proc test(a: AObj) =
+    discard
+
+  let x = AObj()
+  test(x)

tests/misc/t9710.nim

@@ -0,0 +1,6 @@
+discard """
+  matrix: "--debugger:native"
+"""
+# bug #9710
+for i in 1 || 200:
+  discard i