Update tests to use Psbt getters

Author: lukechilds

test/psbt.spec.ts

@@ -523,12 +523,9 @@ describe(`Psbt`, () => {
       });
 
       assert.strictEqual(psbt.inputCount, 1);
-      assert.strictEqual(
-        (psbt as any).__CACHE.__TX.ins[0].sequence,
-        0xffffffff,
-      );
+      assert.strictEqual(psbt.txInputs[0].sequence, 0xffffffff);
       psbt.setInputSequence(0, 0);
-      assert.strictEqual((psbt as any).__CACHE.__TX.ins[0].sequence, 0);
+      assert.strictEqual(psbt.txInputs[0].sequence, 0);
     });
 
     it('throws if input index is too high', () => {