Add unit tests for shortest/longest witness program

Add two unit tests that verify we can correctly determine if a
shortest allowed and longest allowed script is a witness program.

Done in preparation for patching the `witness_version` function.

bitcoin/src/blockdata/script/borrowed.rs

@@ -660,3 +660,31 @@ delegate_index!(
     RangeToInclusive<usize>,
     (Bound<usize>, Bound<usize>)
 );
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::blockdata::script::witness_program::WitnessProgram;
+
+    #[test]
+    fn shortest_witness_program() {
+        let bytes = [0x00; 2]; // Arbitrary bytes, witprog must be between 2 and 40.
+        let version = WitnessVersion::V15; // Arbitrary version number, intentionally not 0 or 1.
+
+        let p = WitnessProgram::new(version, &bytes).expect("failed to create witness program");
+        let script = ScriptBuf::new_witness_program(&p);
+
+        assert_eq!(script.witness_version(), Some(version));
+    }
+
+    #[test]
+    fn longest_witness_program() {
+        let bytes = [0x00; 40]; // Arbitrary bytes, witprog must be between 2 and 40.
+        let version = WitnessVersion::V16; // Arbitrary version number, intentionally not 0 or 1.
+
+        let p = WitnessProgram::new(version, &bytes).expect("failed to create witness program");
+        let script = ScriptBuf::new_witness_program(&p);
+
+        assert_eq!(script.witness_version(), Some(version));
+    }
+}