Use URL test_suite

[vinniefalco]

Now that Boost.URL is in the superproject we can use its testing framework and eliminate the older, redundant copy of the one in JSON:

https://github.com/boostorg/url/tree/develop/extra

[grisumbras]

From #830

Now that Boost.URL is accepted, Boost.JSON can throw out its older iteration of test_suite.hpp and use the one from URL, which is updated to be almost identical in behavior to Boost lightweight test in terms of diagnostic outputs.

For integration tips look to Boost.Http.Proto: https://github.com/CPPAlliance/http_proto/blob/f795e313bee75ca4310f899375227c23f64e1909/test/unit/Jamfile#L12 and https://github.com/CPPAlliance/http_proto/blob/f795e313bee75ca4310f899375227c23f64e1909/test/unit/CMakeLists.txt#L10 .