fix: Anvil builder default port

[MistApproach]

Motivation

Anvil builder new() method docs states the default port to be 8545. In practice, however, current code sets random port number.

Solution

Set default port inside try_spawn() to 8545, unless otherwise specified.
Unit test added.

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[onbjerg]

we actually want it to be assigned to a random port, otherwise the diff instances in tests will conflict entirely which where all of these failures come from

i'd update the docs instead

[MistApproach]

Updated doc-comment for Anvil::new() instead.