Rustdoc json tests: New @hasexact test command
Alot of the time, we wanted to assert that a module had an exact set of items. Most of the time this was done by asserting that the ```@count``` of the module was `n`, and then doing `n` ```@has``` checks on the module.
This was tedious, so often shortcuts were done.
This PR adds a new command to jsondocck to allow consistently expressing this behavior, and then uses it to clean up the tests.
``@rustbot`` modify labels: +A-rustdoc-json +A-testsuite
0491fdad6f