test: refactor test-cluster-send-handle-twice.js

- `var` --> `const` as applicable
- `assert.equal`  --> `assert.strictEqual`
- `assert(false, ..)` --> `common.fail()`
- `common.mustCall` for functions that need to be called exactly once
- modified an `assert(!signal, 'Worker exited by a signal');` call to
  `assert.strictEqual(signal, null);` call as that made more sense

PR-URL: https://github.com/nodejs/node/pull/10049


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>