tests: failure with copied session_spec

[blueyed]

I've noticed that this fails: cp -a test/session{,_new}_spec.lua; make test:

[----------] 24 tests from test/session_new_spec.lua (1645.26 ms total)

[----------] Running tests from test/session_spec.lua
./nvim/msgpack_rpc_stream.lua:1: attempt to index a userdata value

stack traceback:
        ./nvim/msgpack_rpc_stream.lua:1: in main chunk

[----------] 0 tests from test/session_spec.lua (2.01 ms total)

[----------] Global test environment teardown.
[==========] 24 tests from 2 test files ran. (1647.72 ms total)
[  PASSED  ] 24 tests.
[  ERROR   ] 1 error, listed below:
[  ERROR   ] ./nvim/msgpack_rpc_stream.lua @ 1: suite test/session_spec.lua

 1 ERROR
make: *** [Makefile:48: test] Error 1

I.e. when the same spec file is loaded again the local mpack = require('mpack') there fails:

lua-client/nvim/msgpack_rpc_stream.lua

Line 1 in af24091

local mpack = require('mpack')

It's not clear to me what's causing this - it might indicate some inconsistent global state?