test: split out system_ram_tests to signal when total ram cannot be determined

when `GetTotalRAM` returns an `std::nullopt` now we're getting:
```
The following tests did not run:
        106 - system_ram_tests (Skipped)
```

src/test/system_ram_tests.cpp

@@ -0,0 +1,32 @@
+// Copyright (c) 2025-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+#include <common/system.h>
+#include <test/util/setup_common.h>
+
+#include <boost/test/unit_test.hpp>
+
+#include <cstdint>
+#include <optional>
+
+BOOST_AUTO_TEST_SUITE(system_ram_tests)
+
+BOOST_AUTO_TEST_CASE(total_ram)
+{
+    const auto total{GetTotalRAM()};
+    if (!total) {
+        BOOST_WARN_MESSAGE(false, "skipping total_ram: total RAM unknown");
+        return;
+    }
+
+    BOOST_CHECK_GE(*total, 1000_MiB);
+
+    if constexpr (SIZE_MAX == UINT64_MAX) {
+        // Upper bound check only on 64-bit: 32-bit systems can reasonably have max memory,
+        // but extremely large values on 64-bit likely indicate detection errors
+        BOOST_CHECK_LT(*total, 10'000'000_MiB); // >10 TiB memory is unlikely
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()