Remove client arg from test helpers

[kevinAlbs]

Summary

Remove client arg from test helpers that cache results.

Verified with this patch build: https://spruce.mongodb.com/version/67ee96310b754c000700e66c

Motivation

When working on CXX-3209 I was surprised to discover calling test_util::get_max_wire_version on a differently versioned mongocryptd returned the same value as the running mongod:

// Check maxWireVersion of mongocryptd:
mongocxx::client mongocryptd_client(uri("mongodb://localhost:27020"));
auto mwv = test_util::get_max_wire_version(mongocryptd_client); // same as the running mongod?!

The cause appeared to be due to cached results in test helpers:

// These frequently used network calls are cached to avoid bottlenecks during tests.
document::value get_is_master(client const& client) {
    static auto reply = client["admin"].run_command(make_document(kvp("isMaster", 1)));
    return reply;
}

Later calls ignore the client argument. To avoid surprising readers, the client argument is removed from these calls. The calls always use a client constructed with the URI "mongodb://localhost:27017". Overloads accepting a client can be added later as needed.

[ghost]

Nice change!