Remove std::cout and std::cerr from all tests

#### Summary:

Removes `std::cout` and `std::cerr` from Stan's src/test and all subdirectories of src/test.

#### Intended Effect:

This has no effect on code. This replaces all uses of `std::cout` and `std::cerr` with `std::stringstream` with additional checks that the stream is empty at the end of the test.

#### How to Verify:

Tests should be the same. I remember removing one test that was testing `std::cout` directly.

grepping for `std::cout` and `std::cerr` within `src/test` should not return anything.

#### Side Effects:

None. No code within `src/stan` was touched.

#### Documentation:

Not necessary.

#### Reviewer Suggestions:

Anyone. Should be very quick to check.