test/py: Handle u-boot-test-getrole failure

This script can fail if there is no toolchain available for the board.
At present this is not handled very nicely, in that only the error
output is reported. It is much more useful to see everything, so
combine stdout and stderr and report them both.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/py/conftest.py

@@ -165,10 +165,11 @@ def get_details(config):
 	# Make sure the script sees that it is being run from pytest
         env['U_BOOT_SOURCE_DIR'] = source_dir
 
-        proc = subprocess.run(cmd, capture_output=True, encoding='utf-8',
+        proc = subprocess.run(cmd, stdout=subprocess.PIPE,
+                              stderr=subprocess.STDOUT, encoding='utf-8',
                               env=env)
         if proc.returncode:
-            raise ValueError(proc.stderr)
+            raise ValueError(f"Error {proc.returncode} running {cmd}: '{proc.stderr} '{proc.stdout}'")
         # For debugging
         # print('conftest: lab:', proc.stdout)
         vals = {}