Consider using testing.Testing to prevent non-testing purpose

[ccoVeille]

I followed:

• Allow linker to perform dead code elimination for programs using go-cmp #373
• Update readme to mention only using in tests #376

I found these comments very interesting:

• Allow linker to perform dead code elimination for programs using go-cmp #373 (comment)
• Allow linker to perform dead code elimination for programs using go-cmp #373 (comment)

I think testing.Testing could be used to prevent the usage of the lib out of test purpose.

I'm unsure about the implementation. A panic seems an option, but we can be more creative.

It might be a breaking change, but why not after all.

It might help to sort out future issues like:

• Use go-cmp/cmp only in tests kubernetes/kubernetes#104821