separated process execution code into ProcessExecutor

[firewave]

had this sitting around

• refactoring only
• clearly separates the process and thread execution code
• either code is no longer behind a preprocessor define and is visible to more compilers and the various static code analysis (including ours)
• would allow for run-time switching with future changes
• existing tests were just duplicated and should not be redundant
• tests need to be extended to account for differences in implementation (cannot correctly remember what they were but it is at least related to suppression handling)
• easier to define an interface which will leverage the proposed simplifications by @PKEuS
• did some additional iwyu cleanups since I had to run it anyways