Add test_logger as feature to logging (#2586)

## Issue Addressed

Fix #2585

## Proposed Changes

Provide a canonical version of test_logger that can be used
throughout lighthouse.

## Additional Info

This allows tests to conditionally emit logging data by adding
test_logger as the default logger. And then when executing
`cargo test --features logging/test_logger` log output
will be visible:

  wink@3900x:~/lighthouse/common/logging/tests/test-feature-test_logger (Add-test_logger-as-feature-to-logging)
  $ cargo test --features logging/test_logger
      Finished test [unoptimized + debuginfo] target(s) in 0.02s
       Running unittests (target/debug/deps/test_logger-e20115db6a5e3714)

  running 1 test
  Sep 10 12:53:45.212 INFO hi, module: test_logger:8
  test tests::test_fn_with_logging ... ok

  test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Doc-tests test-logger

  running 0 tests

  test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Or, in normal scenarios where logging isn't needed, executing
`cargo test` the log output will not be visible:

  wink@3900x:~/lighthouse/common/logging/tests/test-feature-test_logger (Add-test_logger-as-feature-to-logging)
  $ cargo test
      Finished test [unoptimized + debuginfo] target(s) in 0.02s
       Running unittests (target/debug/deps/test_logger-02e02f8d41e8cf8a)

  running 1 test
  test tests::test_fn_with_logging ... ok

  test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Doc-tests test-logger

  running 0 tests

  test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

common/logging/src/lib.rs

@@ -4,6 +4,7 @@ extern crate lazy_static;
 use lighthouse_metrics::{
     inc_counter, try_create_int_counter, IntCounter, Result as MetricsResult,
 };
+use slog::Logger;
 use slog_term::Decorator;
 use std::io::{Result, Write};
 
@@ -157,3 +158,26 @@ impl<'a> slog_term::RecordDecorator for AlignedRecordDecorator<'a> {
         self.wrapped.start_separator()
     }
 }
+
+/// Return a logger suitable for test usage.
+///
+/// By default no logs will be printed, but they can be enabled via
+/// the `test_logger` feature.  This feature can be enabled for any
+/// dependent crate by passing `--features logging/test_logger`, e.g.
+/// ```bash
+/// cargo test -p beacon_chain --features logging/test_logger
+/// ```
+pub fn test_logger() -> Logger {
+    use sloggers::Build;
+
+    if cfg!(feature = "test_logger") {
+        sloggers::terminal::TerminalLoggerBuilder::new()
+            .level(sloggers::types::Severity::Debug)
+            .build()
+            .expect("Should build test_logger")
+    } else {
+        sloggers::null::NullLoggerBuilder
+            .build()
+            .expect("Should build null_logger")
+    }
+}