tests: framework for testing DepGraph class

This introduces a bespoke fuzzing-focused serialization format for DepGraphs,
and then tests that this format can represent any graph, roundtrips, and then
uses that to test the correctness of DepGraph itself.

This forms the basis for future fuzz tests that need to work with interesting
graphs.

src/Makefile.test.include

@@ -83,6 +83,7 @@ BITCOIN_TESTS =\
   test/bloom_tests.cpp \
   test/bswap_tests.cpp \
   test/checkqueue_tests.cpp \
+  test/cluster_linearize_tests.cpp \
   test/coins_tests.cpp \
   test/coinstatsindex_tests.cpp \
   test/common_url_tests.cpp \
@@ -302,6 +303,7 @@ test_fuzz_fuzz_SOURCES = \
  test/fuzz/buffered_file.cpp \
  test/fuzz/chain.cpp \
  test/fuzz/checkqueue.cpp \
+ test/fuzz/cluster_linearize.cpp \
  test/fuzz/coins_view.cpp \
  test/fuzz/coinscache_sim.cpp \
  test/fuzz/connman.cpp \