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/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(...)