Allow configuring the location of the test directory

Currently, the TCK loads tests from the 'tests' directory within the Node module.

In some cases, it might be beneficial to load tests from a directory outside of the Node module.
To achieve this, I propose adding an environment variable (ASCIIDOC_TCK_TESTS_DIR_PATH) to specify the location of the test directory. If not defined, the tests will be loaded from the 'tests' directory within the Node module by default.

An alternative solution would be to add a command-line option, for example, --tests /path/to/tests-dir.

@mojavelinux What do you think?