Use std::unordered_set instead of std::vector in IsEvicted()

Summary:
An unordered set can tell if an element is present in ~O(1) time (constant on
average, worst case linear to the size of the container), which speeds up and
simplifies the lookup in IsEvicted().

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

This is a backport of [[bitcoin/bitcoin#20197 | core#20197]] [3/6]
bitcoin/bitcoin@ca63b53
Depends on D10976

Test Plan: `ninja check`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10977

src/test/net_peer_eviction_tests.cpp

@@ -10,6 +10,7 @@
 #include <functional>
 #include <numeric>
 #include <optional>
+#include <unordered_set>
 #include <vector>
 
 BOOST_FIXTURE_TEST_SUITE(net_peer_eviction_tests, BasicTestingSetup)
@@ -45,24 +46,23 @@ GetRandomNodeEvictionCandidates(const int n_candidates,
 
 // Returns true if any of the node ids in node_ids are selected for eviction.
 bool IsEvicted(std::vector<NodeEvictionCandidate> candidates,
-               const std::vector<NodeId> &node_ids,
+               const std::unordered_set<NodeId> &node_ids,
                FastRandomContext &random_context) {
     Shuffle(candidates.begin(), candidates.end(), random_context);
     const std::optional<NodeId> evicted_node_id =
         SelectNodeToEvict(std::move(candidates));
     if (!evicted_node_id) {
         return false;
     }
-    return std::find(node_ids.begin(), node_ids.end(), *evicted_node_id) !=
-           node_ids.end();
+    return node_ids.count(*evicted_node_id);
 }
 
 // Create number_of_nodes random nodes, apply setup function candidate_setup_fn,
 // apply eviction logic and then return true if any of the node ids in node_ids
 // are selected for eviction.
 bool IsEvicted(const int number_of_nodes,
                std::function<void(NodeEvictionCandidate &)> candidate_setup_fn,
-               const std::vector<NodeId> &node_ids,
+               const std::unordered_set<NodeId> &node_ids,
                FastRandomContext &random_context) {
     std::vector<NodeEvictionCandidate> candidates =
         GetRandomNodeEvictionCandidates(number_of_nodes, random_context);
@@ -185,7 +185,9 @@ BOOST_AUTO_TEST_CASE(node_eviction_test) {
                     candidate.availabilityScore =
                         double(number_of_nodes - candidate.id);
                 },
-                protectedNodes, random_context));
+                std::unordered_set(protectedNodes.begin(),
+                                   protectedNodes.end()),
+                random_context));
 
             // An eviction is expected given >= 161 random eviction candidates.
             // The eviction logic protects at most four peers by net group,