Allow os_walk to follow symlinks

[ragingpastry]

Allows the testinfra verifier to follow symlinks in the tests
directory.

This fixes a bug described here:
#3021

Previously, os_walk would not follow symlinks. This would break scenarios where you place tests in a common test directory.

For example the following directory structure:

molecule
├── common
│   └── unit
│       ├── test_ldap.py
│       └── test_zabbix.py
├── default
│   ├── conftest.py
│   ├── converge.yml
│   ├── create.yml
│   ├── Dockerfile.j2
│   ├── INSTALL.rst
│   ├── molecule.yml
│   ├── prepare.yml
│   ├── requirements.yml
│   └── tests
│       ├── conftest.py
└── zabbix-4.4.9
    ├── converge.yml
    ├── create.yml
    ├── Dockerfile.j2
    ├── INSTALL.rst
    ├── molecule.yml
    ├── prepare.yml
    ├── requirements.yml
    └── tests
        ├── conftest.py
        └── unit -> ../../common/unit

When running molecule verify -s zabbix-4.4.9 no tests would be found.