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

testing/test-test_logger/src/lib.rs

@@ -0,0 +1,22 @@
+use slog::{info, Logger};
+
+pub struct Config {
+    log: Logger,
+}
+
+pub fn fn_with_logging(config: &Config) {
+    info!(&config.log, "hi");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use logging::test_logger;
+
+    #[test]
+    fn test_fn_with_logging() {
+        let config = Config { log: test_logger() };
+
+        fn_with_logging(&config);
+    }
+}