test/LSP: use less-generic exit code

- os.exit(1) is too generic, since code 1 may be caused by Nvim exiting
  for some other reason.  Change it to os.exit(101).
- style: de-architect json_encode/json_decode calls.

Failure seen in travis macOS job:
https://travis-ci.org/neovim/neovim/jobs/647849133

    [  FAILED  ] test/functional/plugin/lsp_spec.lua@ 266 SP basic_init test should not send didOpen if the buffer closes before init
    test/functional/plugin/lsp_spec.lua:297: exit code
    Expected objects to be the same.
    Passed in:
    (number) 1
    Expected:
    (number) 0
    stack traceback:
      test/functional/plugin/lsp_spec.lua:297: in function 'on_exit'
      test/functional/plugin/lsp_spec.lua💯 in function 'test_rpc_server'
      test/functional/plugin/lsp_spec.lua:272: in function <test/functional/plugin/lsp_spec.lua:266>

test/functional/fixtures/lsp-test-rpc-server.lua

@@ -1,24 +1,5 @@
 local protocol = require 'vim.lsp.protocol'
 
--- Internal utility methods.
-
--- TODO replace with a better implementation.
-local function json_encode(data)
-  local status, result = pcall(vim.fn.json_encode, data)
-  if status then
-    return result
-  else
-    return nil, result
-  end
-end
-local function json_decode(data)
-  local status, result = pcall(vim.fn.json_decode, data)
-  if status then
-    return result
-  else
-    return nil, result
-  end
-end
 
 local function message_parts(sep, ...)
   local parts = {}
@@ -49,16 +30,14 @@ local function format_message_with_content_length(encoded_message)
   }
 end
 
--- Server utility methods.
-
 local function read_message()
   local line = io.read("*l")
   local length = line:lower():match("content%-length:%s*(%d+)")
-  return assert(json_decode(io.read(2 + length):sub(2)), "read_message.json_decode")
+  return vim.fn.json_decode(io.read(2 + length):sub(2))
 end
 
 local function send(payload)
-  io.stdout:write(format_message_with_content_length(json_encode(payload)))
+  io.stdout:write(format_message_with_content_length(vim.fn.json_encode(payload)))
 end
 
 local function respond(id, err, result)
@@ -454,6 +433,6 @@ kill_timer:stop()
 kill_timer:close()
 if not status then
   io.stderr:write(err)
-  os.exit(1)
+  os.exit(101)
 end
 os.exit(0)

test/functional/plugin/lsp_spec.lua

@@ -217,7 +217,7 @@ describe('LSP', function()
           client.stop()
         end;
         on_exit = function(code, signal)
-          eq(1, code, "exit code")
+          eq(101, code, "exit code")
           eq(0, signal, "exit signal")
         end;
         on_callback = function(...)