DevEx: fix `test_logger`

[michaelsproul]

Description

At the moment to get logs in tests requires recompiling with --features logging/test_logger. This is a bit of a waste of time. The test logger also outputs logs to stderr, which evades the Rust test-runner's log capturing (and I think nextest as well).

Perhaps a better solution would be to:

• Always log to stdout when the logger is set to test_logger (which is the case in all tests)
• If necessary, configure aspects of the test_logger via environment variables so no re-compilation is necessary.

This would allow us to run the whole test suite, and then individual tests with --nocapture without recompiling. This might be useful for AI agents wanting to iterate faster as well.

[michaelsproul]

@macladson thoughts?

[michaelsproul]

Rough plan of action:

• Remove test_logger feature from Cargo.toml in logging crate (we will enable test_logger by default)
• Change test_logger to always log to stdout (ignore feature that has been removed)
• Check that this works with running beacon chain tests (no output by default), and with --nocapture (should show output)

[michaelsproul]

@chong-he is going to take this! Thanks CK!