Tests fail for Node.js >= 12

Sharing the logs:
```
✗ same error for bad prime
---
operator: equal
expected: 1
actual: 9
at: Test. (/home/nilesh/ups/diffie-hellman/test.js:127:5)
...
✗ same error for bad prime non testable generator
---
operator: equal
expected: 1
actual: 5
at: Test. (/home/nilesh/ups/diffie-hellman/test.js:130:5)
...
✓ same error for good prime wrong generator
✗ same error for good prime non testable generator
---
operator: equal
expected: 0
actual: 4
at: Test. (/home/nilesh/ups/diffie-hellman/test.js:136:5)
...

tests 233
pass 230
fail 3

Failed Tests: There were 3 failures

✗ check errors same error for bad prime
✗ check errors same error for bad prime non testable generator
✗ check errors same error for good prime non testable generator

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! diffie-hellman@5.0.3 test: `node test.js | tspec`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the diffie-hellman@5.0.3 test script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /home/nilesh/.npm/_logs/2020-01-31T15_53_55_954Z-debug.log

```

まず test.js の 127、130、136 行目にある失敗しているアサーションを確認し、その後、レポートに示されている npm test コマンドを Node.js 12 以降で実行します。3 つのエラーチェックがすべて通過し、完全なテストスイートで失敗なしの 233 個のテスト成功が報告されれば完了です。

索引モデルが issue の本文から書いたものです。