fix(tests): Use a file lock as well as a mutex to isolate tests cases

[aakoshh]

Description

Problem*

The test matrix added in #6429 still causes EOF failures on CI despite #6455 adding a Mutex. This is probably because on CI we use cargo nextest, which runs tests in separate processes (also separate machines because of --partition and matrix, but that shouldn't result in conflicts).

Summary*

Adds a file lock on the Nargo.toml file to coordinate which process is running a specific test.

Additional Context

In my local testing with cargo test -p nargo_cli --test execute the file lock alone wasn't enough, there was always 1 or 2 failures out of the 1600 or so tests, so I left the Mutex in place. Come to think of it, cargo test runs all tests in the same process, so from a POSIX perspective it already has the lock, and when it asks again it doesn't run into conflicts.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.