test_runner: introduce NODE_TEST_WORKER_ID for improved concurrent te…

[cu8code]

ref: #55842

Added a new environment variable, NODE_TEST_WORKER_ID, which ranges from 1 to N when --experimental-test-isolation=process is enabled and defaults to 1 when --experimental-test-isolation=none is used.

Before merging, I want to add some tests but haven't come up with a good approach yet. Here's what I aim to test:

• When --experimental-test-isolation=process is enabled, verify that NODE_TEST_WORKER_ID ranges from 1 to N.
• When --experimental-test-isolation=none is used, ensure that NODE_TEST_WORKER_ID is set to 1.

Any suggestions on how to create such tests would be greatly appreciated!