backcompat_test: improve readability of test failures

Print stdout/err as strings. Print the DTC to stderr instead of logcat
so that it gets forwarded to the main output of test tooling.

Test: atest avf_backcompat_tests
Change-Id: Icf6c46ae057d76163aae4e732381d286a832ffaf
diff --git a/tests/backcompat_test/src/main.rs b/tests/backcompat_test/src/main.rs
index b0cd042..5058e15 100644
--- a/tests/backcompat_test/src/main.rs
+++ b/tests/backcompat_test/src/main.rs
@@ -25,7 +25,6 @@
 use anyhow::anyhow;
 use anyhow::Context;
 use anyhow::Error;
-use log::error;
 use log::info;
 use std::fs::read_to_string;
 use std::fs::File;
@@ -189,15 +188,15 @@
             return Err(anyhow!("failed to execute dtc"));
         }
         let dt2 = read_to_string("dump_dt_failed.dts")?;
-        error!(
+        eprintln!(
             "Device tree 2 does not match golden DT.\n
                Device Tree 2: {}",
             dt2
         );
         return Err(anyhow!(
-            "stdout: {:?}\n stderr: {:?}",
-            dtcompare_res.stdout,
-            dtcompare_res.stderr
+            "stdout: {}\n stderr: {}",
+            String::from_utf8_lossy(&dtcompare_res.stdout),
+            String::from_utf8_lossy(&dtcompare_res.stderr)
         ));
     }