Add tests: nvm run --silent and nvm exec --silent

[ljharb]

I added support for --silent to nvm run and nvm exec in eb81fba for #842.

However, I was lazy, and didn't write any tests.

This issue is a reminder for me, or a request to anyone else, to write these tests. They should ensure that the output is correct without --silent, with --silent, and also that --silent is only swallowed and respected when it's in the third argument position (immediately after run or exec).

[dr15x86]

👍
when i call nvm run --silent /tmp/123.js with not empty .nvmrc
nvm always prompt to stdout Found '/home/..../.nvmrc' with version <stable>

[ljharb]

ah, thank you! I'll fix that.