test: fix flaky child-process-exec-kill-throws

This is a fix for test-child-process-exec-kill-throws which is currently
flaky on Windows.

A bug in the test was causing the child process to fail for reasons
other than those intended by the test. Instead of failing for exceeding
the `maxBuffer` setting, the test was failing because it was trying to
load `internal/child_process` without being passed the
`expose-internals` flag. Move that module to where only the parent
process (which gets the flag) loads it.

Additionally, improve an assertion message to help debug problems like
this.

PR-URL: https://github.com/nodejs/node/pull/12111
Fixes: https://github.com/nodejs/node/issues/12053


Reviewed-By: Richard Lau <riclau@uk.ibm.com>