[MISC] Remove dependency of the seqan3 test_accessor.

[smehringer]

We use the test_accessor to test protected member(function)s and will continue to need this.
Therefore we need to copy it over from seqan3.

Blocked by #7

Tasks:

• copy seqan3/include/core/detail/test_accessor.hpp to sharg/detail/test_accessor.hpp and change the namespace to sharg
• Change all forward declarations to include sharg/detail/test_accessor.hpp instead*
• Switch from befriending the seqan3::detail::test_accessor to sharg::detail::test_accessor in places in inlcude/sharg
• Change the namespace of test_accessor instances in test/unit from seqan3 to sharg
• Check if tests pass
• Where-ever you changed includes in the include/ directory, check whether the file does not inlcude any seqan3 files anymore and tick off the corresponding ones in [FEATURE] Make the argument parser independent of seqan3. #6

*

namespace seqan3::detail
{

struct test_accessor;

} // seqan3::detail