Fuzz test for compact.RangeNodes (#32)

Reading https://github.com/transparency-dev/merkle/blob/main/docs/compact_ranges.md I was inspired to write a simple [fuzz test](https://go.dev/doc/fuzz/) for [compact.RangeNodes](https://pkg.go.dev/github.com/transparency-dev/merkle@v0.0.1/compact#RangeNodes) that checks the property ["a compact range is the minimal set of perfect nodes that cover these, and only these, leaves"](https://github.com/transparency-dev/merkle/blob/main/docs/compact_ranges.md#definition) 

Co-authored-by: Al Cutter <al@google.com>

.github/workflows/go_test.yml

@@ -12,5 +12,5 @@ jobs:
       with:
         go-version: ${{ matrix.go-version }}
     - uses: actions/checkout@v3
-    - run: go test -race -covermode=atomic -coverprofile=coverage.out ./...
+    - run: go test -v -race -covermode=atomic -coverprofile=coverage.out ./...
     - uses: codecov/codecov-action@v2

compact/node_fuzz_test.go

@@ -0,0 +1,33 @@
+//go:build go1.18
+
+package compact
+
+import (
+	"testing"
+)
+
+// Test that RangeNodes returns a slice of nodes with contiguous coverage.
+// https://github.com/transparency-dev/merkle/blob/main/docs/compact_ranges.md#definition
+func FuzzRangeNodes(f *testing.F) {
+	f.Fuzz(func(t *testing.T, begin, end uint64) {
+		if begin > end {
+			return
+		}
+		t.Logf("begin=%d, end=%d", begin, end)
+		nodes := RangeNodes(begin, end, nil)
+		t.Logf("nodes=%v", nodes)
+
+		// Nodes should be contiguous covering begin to end
+		previousEnd := begin
+		for _, node := range nodes {
+			b, e := node.Coverage()
+			if b != previousEnd {
+				t.Errorf("got=%d, want=%d", b, previousEnd)
+			}
+			previousEnd = e
+		}
+		if previousEnd != end {
+			t.Errorf("got=%d, want=%d", previousEnd, end)
+		}
+	})
+}