[5.x] Throw an error when running static:clear when static caching is disabled

[duncanmcclean]

Currently, when you run php please static:clear with static caching disabled, it won't do anything but it'll say the static cache has been cleared, which is misleading if you've just disabled static caching, but still have files in public/static since you'll continue to be served the cached pages.

This pull request mirrors the php please static:warm command by showing a warning if you try to run the command while static caching is disabled.

[jasonvarga]

The tests no longer test anything. The expectations were there intentionally.

They were checking that the command did (or did not) run. It doesn't have to be static:clear, but it should be something.

Maybe the rtfm command would be better than flat:camp. There's no randomness. You could assert on the output.

[duncanmcclean]

They were checking that the command did (or did not) run. It doesn't have to be static:clear, but it should be something.

I've updated the test to call stache:clear instead, since we can do the same kind of assertion with it.

I investigated asserting the output of the rtfm command, however, it'd mean we would need to copy Laravel's PendingCommand class in order to swap out the Kernel import. I was also running into other issues with ->expectsOutput() that I couldn't figure out.

[jasonvarga]

I changed the exit code to 0 to keep it successful. If someone was running static:clear with caching disabled during a deployment for whatever reason, changing to a failure might cause the deployment to fail.

We can change it to a failure in v6.