Add support for silencing only one of the outputs of a test

[Kriskras99]

The old syntax is still supported, the new complete syntax is:

• "{value}": where value is one of [immediate, immediate-final, final, never]
• "{stream}={value}": where {stream} is one of [stdout, stderr]
• "{stream}={value},{stream=value}": where the two {stream} keys cannot be the same

For tests where the output is combined (currently only used for libtest-json output), only the stdout value is used.

There is a small regression where the CLI won't suggest values that are close to the incorrect input, i.e. if you input 'imediate' it won't suggest 'immediate'.

I've not added tests yet, I first wanted to make sure that the implementation is acceptable for inclusion into nextest.

Closes #1688