support/tests: print failed command and output on assertRunOK error

Currently, when asserting that a command succeeded, we just capture the
return code of the command. If that is not zero, the assertion fails,
but the error message is not very splicit:
    AssertionError: 1 != 0

Replace the error message with an explicit message that dumps the failed
command, the error code, and the resulting output.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/infra/basetest.py

@@ -88,7 +88,12 @@ class BRTest(BRConfigTest):
         super(BRTest, self).tearDown()
 
     # Run the given 'cmd' with a 'timeout' on the target and
-    # assert that the command succeeded
+    # assert that the command succeeded; on error, print the
+    # faulty command and its output
     def assertRunOk(self, cmd, timeout=-1):
-        _, exit_code = self.emulator.run(cmd, timeout)
-        self.assertEqual(exit_code, 0)
+        out, exit_code = self.emulator.run(cmd, timeout)
+        self.assertEqual(
+            exit_code,
+            0,
+            "\nFailed to run: {}\noutput was:\n{}".format(cmd, '  '+'\n  '.join(out))
+        )