Use `set -gx` instead of `setenv` for fish shell.

Fix test "activate if the first virtualenv is a virtualenv": it was asserting
the output was of bash form, but not specifying that in the test — it now does.

Fixes https://github.com/pyenv/pyenv-virtualenv/issues/216.

* https://github.com/yyuu/pyenv/pull/615
* https://github.com/fish-shell/fish-shell/issues/310

It was previously evaluated when 'pyenv virtualenv-init' is run.

This makes sure that $VIRTUAL_ENV is handled already in other precmd
functions, especially your prompt.

- fixes `ps` command to lookup parent process
 - adds `assert_output_contains_not` test helper

Ref: https://github.com/yyuu/pyenv/issues/373

This fixes displaying the note, in case "init" has been run manually
and/or the init is not in the expected place.

It removes the unnecessary calls to "grep".

pyenv-virtualenv-init will look at `$PYENV_SHELL` now, too.

Tests have been adjusted/fixed.

When the current pyenv environment is not a virtualenv, this line
exits with a return code of 1. This is annoying for those of us
who report the last return code in our own PROMPT_COMMAND, so
ignore the output of `pyenv activate` if we're not currently using
a virtualenv.

This makes it possible to use `eval "$(pyenv virtualenv-init -)"` from a
function.

"(any command)" is not supported in test in fish, we have to set a
variable and then use that

The situation is when in a directory containing python binary,
`pyenv prefix' will return empty (caused by `pyenv which' returning
"./python"). This change adds quotes for `test', otherwise `test' will
warn about missing argument.

$ pyenv global system
    $ pyenv shell venv
    (venv) $ pyenv shell --unset
    (venv) $ pyenv version
    system (set by /home/yamashita/.pyenv/version)
    (venv) $ pyenv which python
    /home/yamashita/.pyenv/versions/venv/bin/python # <-- should be `/usr/bin/python` (system)