Alcotezt-UX: fix invocation headers II

Related to issue #5172 (closed)

Invocation headers

• Test files have traditionally contained a "test header", as described here: https://tezos.gitlab.io/developer/testing.html#conventions, including an invocation header.
• During the project, we've broken most of the invocation headers. Here's an example of a broken header: https://gitlab.com/tezos/tezos/blob/master/src/proto_016_PtMumbai/lib_client/test/test_client_proto_context.ml#L29

The current fix is to rewrite it as:

dune exec [TEST_FOLDER]/main.exe

• lib_requester/test
• lib_rpc_http/test
• lib_shell/test
• lib_shell_services/test
• lib_signer_backends/test
• lib_signer_backends/unix/test
• lib_stdlib/test
• lib_stdlib/test-unix
• lib_stdlib_unix/test
• lib_workers/test