activate.fish erases a pre-existing NODE_PATH and never restores it

[ekalinin]

Summary

Sourcing bin/activate.fish destroys a NODE_PATH the user already had
set. deactivate_node then has nothing to restore, so the variable is gone
for good. NPM_CONFIG_PREFIX and npm_config_prefix have the same defect.

Reproduction

$ nodeenv --prebuilt /tmp/ne
$ fish -c 'set -gx NODE_PATH /my/node_path
           source /tmp/ne/bin/activate.fish
           deactivate_node
           echo "NODE_PATH=[$NODE_PATH]"'
NODE_PATH=[]

Expected NODE_PATH=[/my/node_path], which is what the POSIX bin/activate
does under sh, dash and bash.

Cause

activate.fish calls deactivate_node nondestructive near the top, before
the activation code has saved anything:

# unset irrelevant variables
deactivate_node nondestructive

On that pass _OLD_NODE_PATH is empty, so deactivate_node takes its else
branch and erases the variable:

if test -n "$_OLD_NODE_PATH"
    set -gx NODE_PATH $_OLD_NODE_PATH
    set -e _OLD_NODE_PATH
else
    set -e NODE_PATH
end

The activation code that runs afterwards therefore sees set -q NODE_PATH
as false, never records _OLD_NODE_PATH, and the user's value is lost:

if set -q NODE_PATH
    set -gx _OLD_NODE_PATH $NODE_PATH
    set -gx NODE_PATH "$NODE_VIRTUAL_ENV/lib/node_modules" $NODE_PATH
else
    set -gx NODE_PATH "$NODE_VIRTUAL_ENV/lib/node_modules"
end

b042056 ("fix(nodeenv): guard fish npm config restore") already fixed
exactly this disease, but only for the --isolate-npm variables, by
wrapping the restore in a guard:

# Skip the "deactivate_node nondestructive" pass at the top of
# activate.fish: the variables are only saved after it has run
if set -q NODE_VIRTUAL_ENV

NODE_PATH and the NPM_CONFIG_PREFIX / npm_config_prefix pair never got
that guard.

Scope of the fix

All three variable groups, not just NODE_PATH. The prefix pair is affected
identically; it only looks fine today because the test asserts NODE_PATH
first and stops there.

Test coverage

tests/test_activate_shells.py (added in #397) marks this
xfail(strict=True) as FISH_NODE_PATH_CLOBBER on
test_deactivate_restores_env, via the restoring_shell decorator. When the
fix lands, drop the constant and the decorator and put the test back on
@activating_shell.