websocket: fix curl_ws_recv()

- when data arrived in several chunks, the collection into
  the passed buffer always started at offset 0, overwriting
  the data already there.

adding test_20_07 to verify fix

- debug environment var CURL_WS_CHUNK_SIZE can be used to
  influence the buffer chunk size used for en-/decoding.

Closes #12945

tests/http/testenv/client.py

@@ -45,10 +45,12 @@ log = logging.getLogger(__name__)
 class LocalClient:
 
     def __init__(self, name: str, env: Env, run_dir: Optional[str] = None,
-                 timeout: Optional[float] = None):
+                 timeout: Optional[float] = None,
+                 run_env: Optional[Dict[str,str]] = None):
         self.name = name
         self.path = os.path.join(env.project_dir, f'tests/http/clients/{name}')
         self.env = env
+        self._run_env= run_env
         self._timeout = timeout if timeout else env.test_timeout
         self._curl = os.environ['CURL'] if 'CURL' in os.environ else env.curl
         self._run_dir = run_dir if run_dir else os.path.join(env.gen_dir, name)
@@ -95,7 +97,7 @@ class LocalClient:
                 with open(self._stderrfile, 'w') as cerr:
                     p = subprocess.run(myargs, stderr=cerr, stdout=cout,
                                        cwd=self._run_dir, shell=False,
-                                       input=None,
+                                       input=None, env=self._run_env,
                                        timeout=self._timeout)
                     exitcode = p.returncode
         except subprocess.TimeoutExpired: