make `collector/internal/testdata` available as a public utility package

[codeboten]

Right now this package being included in go.opentelemetry.io/collector means that anyone using the package for its tests depends on all the packages the collector package depends on.

Making the package a public package with no intention of stabilizing it means that we can at least untangle those dependencies.

[codeboten]

Alternatively, i could just copy the test files in the places where i dont want to incur dependencies :) but it would be convenient not to

[codeboten]

Another alternative is to make this a package under pdata/testdata

[atoulme]

I vote to make it available under testdata and create enough documentation that implementers downstream can leverage this package.

[dmitryax]

I don't like moving it to pdata/testdata as it is given that it's 1.x. I think we should better assess the interface to make it cleaner and extensible in the future. For example, each Generate... function probably needs to accept some settings struct instead of just one number

[dmitryax]

UPD: Oh, I see you moved it as another 0.x module on top of pdata. I think that should be ok