tests: fix tests with npm7

[AdamMajer]

When run with npm 7.x, the unit tests fails with the error below.
To fix this, simply don't change the CWD when executing the tests

pm2@4.5.0 test
bash test/unit.sh && bash test/e2e.sh

[~] Starting test ./filter_env.mocha.js
npm ERR! code ENOENT
npm ERR! syscall open
npm ERR! path /home/adamm/work/node/pm2/pm2/test/programmatic/package.json
npm ERR! errno -2
npm ERR! enoent ENOENT: no such file or directory, open '/home/adamm/work/node/pm2/pm2/test/programmatic/package.json'
npm ERR! enoent This is related to npm not being able to find a file.
npm ERR! enoent

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	---
License	MIT
Doc PR	https://github.com/pm2-hive/pm2-hive.github.io/pulls

[CLAassistant]

All committers have signed the CLA.

[AdamMajer]

FWIW, the "signed CLA" is not really "signed"... GitHub interface is complete junk for this --- "Sign in with GitHub to Continue" and never a confirmation..... so it's great :)

If you need CLA signed, it should be done by my company.