test: use default assertion message

In test-child-process-spawnsync, the assert.strictEqual() custom
message was hiding information about why the test has failed. It
just showed what value is expected and in case of failure we want to
know which value has caused test to fail.

PR-URL: https://github.com/nodejs/node/pull/16819


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>