Appveyor thttpclient (#6744)

* App option value for disabling tests for AppVeyor

* Disable thttpclient on AppVeyor

tests/stdlib/thttpclient.nim

@@ -3,6 +3,7 @@ discard """
   exitcode: 0
   output: "OK"
   disabled: "travis"
+  disabled: "appveyor"
 """
 
 import strutils

tests/testament/specs.nim

@@ -13,6 +13,7 @@ import parseutils, strutils, os, osproc, streams, parsecfg
 var compilerPrefix* = "compiler" / "nim "
 
 let isTravis = existsEnv("TRAVIS")
+let isAppVeyor = existsEnv("APPVEYOR")
 
 proc cmdTemplate*(): string =
   compilerPrefix & "$target --lib:lib --hints:on -d:testing $options $file"
@@ -178,6 +179,8 @@ proc parseSpec*(filename: string): TSpec =
         when defined(posix): result.err = reIgnored
       of "travis":
         if isTravis: result.err = reIgnored
+      of "appveyor":
+        if isAppVeyor: result.err = reIgnored
       else:
         raise newException(ValueError, "cannot interpret as a bool: " & e.value)
     of "cmd":