test: refactor test-child-process-stdin

Use assert.strictEqual instead of assert.equal and assert.ok

PR-URL: https://github.com/nodejs/node/pull/10420


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>