metamorphic: fix TestOptionsRoundtrip for TombstoneDenseCompactionThreshold

Add TombstoneDenseCompactionThreshold to the list of ignored function
pointer fields in TestOptionsRoundtrip, and add a check to verify the
function's return value matches after serialization/deserialization.

This fixes test failures that occurred because function pointers differ
after roundtrip serialization, even though the returned values are
identical. The fix follows the existing pattern used for other function-
typed options like EnableValueBlocks and DisableIngestAsFlushable.

Author: ever0de

metamorphic/options_test.go

@@ -85,6 +85,7 @@ func TestOptionsRoundtrip(t *testing.T) {
 		"Experimental.RemoteStorage:",
 		"Experimental.SingleDeleteInvariantViolationCallback:",
 		"Experimental.EnableDeleteOnlyCompactionExcises:",
+		"Experimental.TombstoneDenseCompactionThreshold:",
 		"Experimental.ValueSeparationPolicy:",
 		"Levels[0].Compression:",
 		"Levels[1].Compression:",
@@ -116,6 +117,7 @@ func TestOptionsRoundtrip(t *testing.T) {
 		expectEqualFn(t, o.Opts.Experimental.DisableIngestAsFlushable, parsed.Opts.Experimental.DisableIngestAsFlushable)
 		expectEqualFn(t, o.Opts.Experimental.IngestSplit, parsed.Opts.Experimental.IngestSplit)
 		expectEqualFn(t, o.Opts.Experimental.CompactionGarbageFractionForMaxConcurrency, parsed.Opts.Experimental.CompactionGarbageFractionForMaxConcurrency)
+		expectEqualFn(t, o.Opts.Experimental.TombstoneDenseCompactionThreshold, parsed.Opts.Experimental.TombstoneDenseCompactionThreshold)
 		expectEqualFn(t, o.Opts.Experimental.ValueSeparationPolicy, parsed.Opts.Experimental.ValueSeparationPolicy)
 		expectEqualFn(t, o.Opts.TargetByteDeletionRate, parsed.Opts.TargetByteDeletionRate)