test(unordered_lru_cache): add comprehensive unit tests

Add Boost.Test suite with 17 test cases covering:
- Construction and configuration (runtime/compile-time sizing)
- Basic API behavior (insert, get, emplace, exists, erase, clear)
- LRU and truncation semantics (threshold behavior, recency tracking)
- Edge cases (maxSize=1, various threshold combinations)

Also optimize truncate_if_needed() to use std::nth_element instead
of std::sort for O(N) average complexity instead of O(N log N).

Author: PastaPastaPasta

src/Makefile.test.include

@@ -196,6 +196,7 @@ BITCOIN_TESTS =\
   test/txvalidation_tests.cpp \
   test/txvalidationcache_tests.cpp \
   test/uint256_tests.cpp \
+  test/unordered_lru_cache_tests.cpp \
   test/util_tests.cpp \
   test/validation_block_tests.cpp \
   test/validation_chainstate_tests.cpp \