[lldb][test] Print command output when it unexpectedly passes (#224021)
Some LLDB tests expect commands to fail. However, we do sometimes see
these commands actually pass on the bots and the test output doesn't
really indicate what is going on.
This patch adds the (error) output of commands that we expected to fail
but didn't do the test output. This changes the dreaded `Command 'foo'
is expected to fail!` error to:
```
Command 'foo' is expected to fail!
Output: bar
```