test: test private key must be defined

This test was excluded because the precondition wasn't hit when calling
`keyring.onDecrypt` (I think due to another precondition somewhere
else).

It might be a bit sneaky to call the "private" method (`_unwrapKey`) in
the tests, but it hits the precondition we're trying to test more
directly.

modules/raw-rsa-keyring-node/test/raw_rsa_keyring_node.test.ts

@@ -168,7 +168,7 @@ describe('RawRsaKeyringNode encrypt/decrypt', () => {
     return expect(keyring.onEncrypt(material)).to.rejectedWith(Error)
   })
 
-  xit('Precondition: Private key must be defined to support decrypt.', async () => {
+  it('Precondition: Private key must be defined to support decrypt.', async () => {
     const keyring = new RawRsaKeyringNode({
       rsaKey: { publicKey: publicPem },
       keyName,
@@ -177,7 +177,6 @@ describe('RawRsaKeyringNode encrypt/decrypt', () => {
 
     const suite = new NodeAlgorithmSuite(AlgorithmSuiteIdentifier.ALG_AES256_GCM_IV12_TAG16_HKDF_SHA256)
     const material = new NodeDecryptionMaterial(suite, {})
-    await keyring.onDecrypt(material, [encryptedDataKey])
-    return expect(keyring.onDecrypt(material, [encryptedDataKey])).to.rejectedWith(Error)
+    return expect(keyring._unwrapKey(material, encryptedDataKey)).to.rejectedWith(Error)
   })
 })