Add unit testing, stabilize the container and image listing

tests/test_container_parser.cpp

@@ -0,0 +1,27 @@
+#include "container_parser.hpp"
+
+#include <cassert>
+#include <iostream>
+
+int main() {
+    const auto result = parseContainerList(R"([
+      {"Id":"running-id","Names":["/web"],"State":"running","Status":"Up 2 minutes","Ports":[]},
+      {"Id":"exited-id","Names":["/old"],"State":"exited","Status":"Exited (0) 1 minute ago","Ports":[{"PrivatePort":80,"Type":"tcp"}]},
+      {"Id":"odd-id","Names":["/odd"],"State":"exited","Ports":"unexpected"}
+    ])");
+    assert(result.ok());
+    assert(result.items.size() == 3);
+    assert(result.items[0].running);
+    assert(!result.items[1].running);
+    assert(result.items[1].name == "old");
+    assert(result.items[1].ports.empty());
+    assert(result.items[2].name == "odd");
+
+    const auto empty = parseContainerList("[]");
+    assert(empty.ok());
+    assert(empty.items.empty());
+
+    const auto invalid = parseContainerList("not-json");
+    assert(!invalid.ok());
+    std::cout << "Container parser tests passed\n";
+}