nixos/tests: fix succeed() with multiple commands

we previously immediately returned the first commands output, and didn't
execute any of the other commands.

Now, return the last commands output.
This should be documented in the method docstring.

nixos/lib/test-driver/test-driver.py

@@ -381,14 +381,16 @@ class Machine:
 
     def succeed(self, *commands):
         """Execute each command and check that it succeeds."""
+        output = ""
         for command in commands:
             with self.nested("must succeed: {}".format(command)):
-                status, output = self.execute(command)
+                (status, out) = self.execute(command)
                 if status != 0:
-                    self.log("output: {}".format(output))
+                    self.log("output: {}".format(out))
                     raise Exception(
                         "command `{}` failed (exit code {})".format(command, status)
                     )
+                output += out
         return output
 
     def fail(self, *commands):