support/testing/infra: improve run_cmd_on_host() to show stdout/stderr

When run_cmd_on_host() runs a command that fails, we only get an
exception with no details to debug what happened. Let's improve that
by catching the exception, and printing the command output. This
requires redirecting stderr to stdout (instead of /dev/null) and
asking to get the output in text format.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

support/testing/infra/__init__.py

@@ -59,11 +59,20 @@ def download(dldir, filename):
 
 def run_cmd_on_host(builddir, cmd):
     """Call subprocess.check_output and return the text output."""
-    out = subprocess.check_output(cmd,
-                                  stderr=open(os.devnull, "w"),
-                                  cwd=builddir,
-                                  env={"LANG": "C"},
-                                  universal_newlines=True)
+    try:
+        out = subprocess.check_output(cmd,
+                                      cwd=builddir,
+                                      env={"LANG": "C"},
+                                      stderr=subprocess.STDOUT,
+                                      text=True,
+                                      universal_newlines=True)
+    except subprocess.CalledProcessError as e:
+        print(f"Command failed with return code {e.returncode}")
+        print("=== STDOUT/STDERR ===")
+        print(e.output)
+        print("=====================")
+        raise
+
     return out