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/Cargo.toml

@@ -4,8 +4,12 @@ version = "0.2.0"
 authors = ["blacktemplar <blacktemplar@a1.net>"]
 edition = "2018"
 
+[features]
+test_logger = [] # Print log output to stderr when running tests instead of dropping it
+
 [dependencies]
 slog = "2.5.2"
 slog-term = "2.6.0"
 lighthouse_metrics = { path = "../lighthouse_metrics" }
 lazy_static = "1.4.0"
+sloggers = "2.0.2"

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")
+    }
+}

common/logging/tests/test.rs

@@ -0,0 +1,53 @@
+use std::env;
+use std::process::Command;
+use std::process::Output;
+
+fn run_cmd(cmd_line: &str) -> Result<Output, std::io::Error> {
+    let output;
+    if cfg!(target_os = "windows") {
+        output = Command::new(r#"cmd"#).args(["/C", cmd_line]).output();
+    } else {
+        output = Command::new(r#"sh"#).args(["-c", cmd_line]).output();
+    }
+    output
+}
+
+#[test]
+fn test_test_logger_with_feature_test_logger() {
+    let cur_dir = env::current_dir().unwrap();
+    let test_dir = cur_dir
+        .join("..")
+        .join("..")
+        .join("testing")
+        .join("test-test_logger");
+    let cmd_line = format!(
+        "cd {} && cargo test --features logging/test_logger",
+        test_dir.to_str().unwrap()
+    );
+
+    let output = run_cmd(&cmd_line);
+
+    // Assert output data DOES contain "INFO hi, "
+    let data = String::from_utf8(output.unwrap().stderr).unwrap();
+    println!("data={}", data);
+    assert!(data.contains("INFO hi, "));
+}
+
+#[test]
+fn test_test_logger_no_features() {
+    // Test without features
+    let cur_dir = env::current_dir().unwrap();
+    let test_dir = cur_dir
+        .join("..")
+        .join("..")
+        .join("testing")
+        .join("test-test_logger");
+    let cmd_line = format!("cd {} && cargo test", test_dir.to_str().unwrap());
+
+    let output = run_cmd(&cmd_line);
+
+    // Assert output data DOES contain "INFO hi, "
+    let data = String::from_utf8(output.unwrap().stderr).unwrap();
+    println!("data={}", data);
+    assert!(!data.contains("INFO hi, "));
+}