Comment out flaky test in tests/stdlib/thttpclient (#23772)

```
$ curl -v http://example.com/404 |& grep 'HTTP/1.1'
> GET /404 HTTP/1.1
< HTTP/1.1 500 Internal Server Error
```

So, the test with http://example.com/404 should be disabled, I think.

tests/stdlib/thttpclient.nim

@@ -53,9 +53,9 @@ proc asyncTest() {.async.} =
   doAssert("<title>Example Domain</title>" in body)
 
   resp = await client.request("http://example.com/404")
-  doAssert(resp.code.is4xx)
-  doAssert(resp.code == Http404)
-  doAssert(resp.status == $Http404)
+  doAssert(resp.code.is4xx or resp.code.is5xx)
+  doAssert(resp.code == Http404 or resp.code == Http500)
+  doAssert(resp.status == $Http404 or resp.status == $Http500)
 
   when false: # occasionally does not give success code 
     resp = await client.request("https://google.com/")
@@ -115,9 +115,9 @@ proc syncTest() =
   doAssert("<title>Example Domain</title>" in resp.body)
 
   resp = client.request("http://example.com/404")
-  doAssert(resp.code.is4xx)
-  doAssert(resp.code == Http404)
-  doAssert(resp.status == $Http404)
+  doAssert(resp.code.is4xx or resp.code.is5xx)
+  doAssert(resp.code == Http404 or resp.code == Http500)
+  doAssert(resp.status == $Http404 or resp.status == $Http500)
 
   when false: # occasionally does not give success code
     resp = client.request("https://google.com/")