These tests are essentially affirming a regression vs Vim. In Vim,
    :echo system('cat - &', 'foo')
returns "foo", because Vim internally wraps the command with shell-specific
syntax to redirect the streams from /dev/null[1].

That can't work in Nvim because we use pipes directly (instead of temp files)
and don't wrap the command with shell-specific redirection syntax.

References #3529
References #5241

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_03_02

When skipping these test blocks they may error out:

    Error → test/functional/shell/viml_system_spec.lua @ 154
    system() with output containing NULs setup
    ./test/functional/helpers.lua:75: attempt to index upvalue 'session' (a nil value)

    stack traceback:
            ./test/functional/helpers.lua:75: in function 'request'
            ./test/functional/helpers.lua:166: in function 'nvim_feed'
            ./test/functional/helpers.lua:195: in function 'feed'
            test/functional/shell/viml_system_spec.lua:14: in function <test/functional/shell/viml_system_spec.lua:13>

    Error → test/functional/shell/viml_system_spec.lua @ 155
    system() with output containing NULs teardown
    ./test/functional/helpers.lua:75: attempt to index upvalue 'session' (a nil value)

    stack traceback:
            ./test/functional/helpers.lua:75: in function 'eval'
            test/functional/shell/viml_system_spec.lua:21: in function <test/functional/shell/viml_system_spec.lua:20>

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

Setting `msg_didout` after `call_shell` is enough as it will cause
`hit_return_msg()` to print on next line.

Close #3269

AFAICT busted does not report pending() invocations without the 2nd
argument.

For any of these functions, if {cmd} is a string, execute
"&shell &shellcmdflag '{cmd}'", or simply {cmd} if it's a list.

In termopen(), if the 'name' option is not supplied, try to guess using
'{cmd}' (string) or {cmd}[0] (list).  Simplify ex_terminal to use the
string form of termopen().

termopen: get name from argument

Convert list_to_argv to tv_to_argv.

Helped-by: Björn Linse <@bfredl>
Helped-by: oni-link <knil.ino@gmail.com>
Helped-by: Thiago de Arruda <@tarruda>