checkResult() should be optional

[vorburger]

The waitForExit() methods currently internally (all?) use checkResult().

This makes it impossible to wait for a non-sucessful exit - which could be handy for some use cases (e.g. a CLI testing tool).

[mihneagiurgea]

I think this is potentially related: #47

[vorburger]

@mihneagiurgea your #47 is a "bug" where an external process which is expected to exit with 0 (mysqld) is handled wrong. I suspect #96 actually fixed that problem.

This issue however is an "enhancement" to make this library usable to exec something which one is expecting (!) to have a non-zero exit code. One example of this is my use of this library for the Executable Markdown of Enola.dev, notably to avoid this (very) ugly initial workwaround.