activate: display setup instructions only with PYENV_VIRTUALENV_INIT=0

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.

bin/pyenv-virtualenv-init

@@ -18,7 +18,7 @@ do
   fi
 done
 
-shell="$1"
+shell="${1:-$PYENV_SHELL}"
 if [ -z "$shell" ]; then
   shell="$(ps c -p "$PPID" -o 'ucomm=' 2>/dev/null || true)"
   shell="${shell##-}"