lua-nucleo/test/cases/0290-tstr: fix compatibility with LuaJIT 2

[rafis]

Running test under LuaJIT 2.0.2 gives error:

Suite test  tstr_cat-string
lua-nucleo/ensure.lua:216: ensure_strequals: tstr zero symbol:
different at byte 12 (line 1, offset 1):

  expected   |embedded\000zero"|
vs. actual   |embedded\0zero"|

expected has 2 extra characters
actual:
"embedded\0zero"
expected:
"embedded\000zero"
stack traceback:
    [C]: in function 'error'
    lua-nucleo/ensure.lua:216: in function 'ensure_strequals'
    test/cases/0290-tstr.lua:64: in function 'check_ok'
    test/cases/0290-tstr.lua:73: in function 'check_str_ok'
    test/cases/0290-tstr.lua:88: in function 'fn'
    lua-nucleo/suite.lua:167: in function 
    [C]: in function 'xpcall'
    lua-nucleo/suite.lua:167: in function 'run'
    lua-nucleo/suite.lua:556: in function 
    [C]: in function 'xpcall'
    lua-nucleo/suite.lua:551: in function 'run_test'
    lua-nucleo/suite.lua:600: in function 'run_tests'
    test/test.lua:134: in main chunk
    [C]: at 0x00404980
ERR 

#2377