test/py: Provide a way to check that a command fails

Sometimes we want to run a command and check that it fails. Add a function
to handle this. It can check the return code and also make sure that the
output contains a given error message.

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

test/py/u_boot_utils.py

@@ -185,6 +185,28 @@ def cmd(u_boot_console, cmd_str):
     """
     return run_and_log(u_boot_console, cmd_str.split())
 
+def run_and_log_expect_exception(u_boot_console, cmd, retcode, msg):
+    """Run a command which is expected to fail.
+
+    This runs a command and checks that it fails with the expected return code
+    and exception method. If not, an exception is raised.
+
+    Args:
+        u_boot_console: A console connection to U-Boot.
+        cmd: The command to run, as an array of argv[].
+        retcode: Expected non-zero return code from the command.
+        msg: String which should be contained within the command's output.
+    """
+    try:
+        runner = u_boot_console.log.get_runner(cmd[0], sys.stdout)
+        runner.run(cmd)
+    except Exception as e:
+        assert(msg in runner.output)
+    else:
+        raise Exception('Expected exception, but not raised')
+    finally:
+        runner.close()
+
 ram_base = None
 def find_ram_base(u_boot_console):
     """Find the running U-Boot's RAM location.