sim: Enable logging in simulator test cases

When running simulations as unit tests, use a workaround from
https://stackoverflow.com/questions/30177845/how-to-initialize-the-logger-for-integration-tests
to initialize the logging system.

Signed-off-by: David Brown <david.brown@linaro.org>

sim/src/lib.rs

@@ -21,6 +21,7 @@ use std::slice;
 
 mod caps;
 mod tlv;
+pub mod testlog;
 
 use simflash::{Flash, SimFlash};
 use mcuboot_sys::{c, AreaDesc, FlashId};

sim/src/testlog.rs

@@ -0,0 +1,19 @@
+//! Logging support for the test framework.
+//!
+//! https://stackoverflow.com/questions/30177845/how-to-initialize-the-logger-for-integration-tests
+//!
+//! The test framework runs the tests, possibly simultaneously, and in various orders.  This helper
+//! function, which should be called at the beginning of each test, will setup logging for all of
+//! the tests.
+
+use env_logger;
+use std::sync::{Once, ONCE_INIT};
+
+static INIT: Once = ONCE_INIT;
+
+/// Setup the logging system.  Intended to be called at the beginning of each test.
+pub fn setup() {
+    INIT.call_once(|| {
+        env_logger::init().unwrap();
+    });
+}

sim/tests/core.rs

@@ -5,9 +5,12 @@
 extern crate bootsim;
 
 use bootsim::{ALL_DEVICES, RunStatus};
+use bootsim::testlog;
 
 #[test]
 fn core_tests() {
+    testlog::setup();
+
     let mut status = RunStatus::new();
 
     for &dev in ALL_DEVICES {