add test for define, undef

[timotheecour]

undef is actually not used at all in nim repo, but I rely on it; so I'm adding a test

[kaushalmodi]

TIL undef exists :)

[timotheecour]

exactly. that's the point :)
btw @Araq I found the following pattern extremely useful when debugging, for example, the compiler:

proc problematicCode =
  {.define(enableMyDebug).}
  codeThatNeedsCompilerDebuggingEnabled()
  {.undef(enableMyDebug).}

# in compiler/debugs.nim
macro myDebug(...) =
  let conf = getConfGlobal() # global var set in newConfigRef(), when debugging enabled
  if isDefined(conf, "enableDebug"):
    # pretty print all arguments, with line numbers etc using https://github.com/nim-lang/RFCs/issues/203#issuecomment-602534906
  else: # do nothing

# somewhere inside compiler
proc semExpr() =
  ...
  myDebug t, n, n2.renderTree # compiler code temporarily littered with these while debugging something
  ...

this allows showing debug logs only for the section you're interested in
it's more useful in particular than msgs.??:

proc `??`* (conf: ConfigRef; info: TLineInfo, filename: string): bool =
  # only for debugging purposes
  result = filename in toFilename(conf, info)