Lib_proxy_server_config: remove double runtest and restrict tests

Context

The tests in Lib_proxy_server_config where not restricted to that package.

Furthermore, it seems tests are doubly defined in tezos-proxy-server-config: once with a (tests stanza and once with a (rule (alias runtest: this has the effect of running the tests twice if you do dune runtest src/lib_proxy_server_config/.

This MR restricts the tests to their package and removes the doubly defined runtest.

Manually testing the MR

Run:

dune runtest src/lib_proxy_server_config/

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• n/a Provide automatic testing (see the testing guide).
• n/a For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.