Tests fail for Node.js >= 12

Nobody has claimed this yet.

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

```

Start with the failing assertions in test.js at lines 127, 130, and 136, then run the npm test command shown in the report under Node.js 12 or newer. Done means the three error checks pass and the full suite reports 233 passing tests with no failures.

Written by the indexing model from the issue text.