patch 8.2.4780: parsing an LSP message fails when it is split

Problem:    Parsing an LSP message fails when it is split.
Solution:   Collapse the received data before parsing. (Yegappan Lakshmanan,
            closes #10215)
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index e270a4b..6fef421 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -2580,6 +2580,11 @@
   call assert_equal({'id': 14, 'jsonrpc': '2.0', 'result': 'extra-hdr-fields'},
         \ resp)
 
+  " Test for processing delayed payload
+  let resp = ch_evalexpr(ch, #{method: 'delayed-payload', params: {}})
+  call assert_equal({'id': 15, 'jsonrpc': '2.0', 'result': 'delayed-payload'},
+        \ resp)
+
   " Test for processing a HTTP header without the Content-Length field
   let resp = ch_evalexpr(ch, #{method: 'hdr-without-len', params: {}},
         \ #{timeout: 200})
@@ -2629,13 +2634,6 @@
   call assert_equal([], g:lspNotif)
   " Restore the callback function
   call ch_setoptions(ch, #{callback: 'LspCb'})
-  let g:lspNotif = []
-  call ch_sendexpr(ch, #{method: 'echo', params: #{s: 'no-callback'}})
-  " Send a ping to wait for all the notification messages to arrive
-  call assert_equal('alive', ch_evalexpr(ch, #{method: 'ping'}).result)
-  call assert_equal([#{jsonrpc: '2.0', result:
-        \ #{method: 'echo', jsonrpc: '2.0', params: #{s: 'no-callback'}}}],
-        \ g:lspNotif)
 
   " " Test for sending a raw message
   " let g:lspNotif = []