Add tests for ClipboardContext and NopClipboardContext

[yykamei]

copypasta doesn't seem to have unit tests, so I accidentally made changes with the PR: #57

I want to make sure any changes are checked by running test codes. That's why I want to add test codes.

[chrisduerr]

There are no tests for the exact reason the CI is failing this build.

These tests also would not have helped with the issue you mentioned at all.

[yykamei]

OK, I understood this PR doesn't help the issue I mentioned. What should I do to avoid the issue?

[kchibisov]

I mean, you could test for object safety, obviously, which is a completely different test and it doesn't require anything special.

It seems like you haven't understood what the exact reason was why I reverted the change. Basically rust can't have Box<dyn Trait> trait objects where Trait has a generic on any of its methods, so that's why your case doesn't work. What you should do instead is to basically build the clipboard and put it into the Box<dyn ClipboardProvider>, no need to actually set_contents, etc.

Just testing with Nop clipboard on its own should be more than enough. though or with doc tests. Actual building won't work.

[yykamei]

Thank you for your explanation.