test: add stdio checks to cp-exec-maxBuffer

Expands this test case to check what happens to stdout/stderr when
maxBuffer is exceeded.

Also changes how cases are checked so that assertion stacks are
tracable to their test case, aka 'make it actually debuggable'.

PR-URL: https://github.com/nodejs/node/pull/24951


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>