What happens in tests when the requested port is in use? What's verified?

When a requested port is in use, the tests verify the fallback behavior. Here are the key test cases:

Single preferred port unavailable [test.js:L23-L30]: When a desired port (8282) is already bound by a server, getPort() returns a different available port instead. The test verifies that:

• The returned port is a number greater than 0
• The returned port is not the unavailable desired port

Multiple preferred ports unavailable [test.js:L57-L75]: When the first port in an array of preferred ports (9090) is occupied, getPort() tries the next one (9091). The test verifies:

• If the first port is unavailable, it falls back to the second port in the array
• If all ports in the array are unavailable, it returns a random available port that is:
  • A valid port number (between 0 and 65,536)
  • Not any of the unavailable desired ports

IPv4 host-specific test [test.js:L60-L67]: When checking with a specific host (0.0.0.0), if the first preferred port is taken, the function still returns the next available port from the preferred list.

The core pattern is: the function gracefully degrades from preferred ports to a randomly allocated available port when the preferred ones are in use.

FOLLOW_UPS:

• How does the reserve option affect port locking behavior?
• What validation errors can the exclude parameter throw?
• How do iterables work as preferred port inputs?