test: fix reading STDIN command on openSUSE

Inspired by gh-5064, that breaks the previous version of the test on
openSUSE. When using `io.popen:write()` on tarantool with `-i` flag, it
failed to run the command on openSUSE. This happened because before
gh-5064 patch it used to employ `luaL_loadfile()` that interprets EOF
as the end of the command, while when it is loaded as a string openSUSE
expects it to end with '\n'.

Needed for #5064
NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix