fuzz: Link BasicTestingSetup (shared with unit tests) #15504

Link against BasicTestingSetup in the fuzz tests, so we can fuzz against validation.

Also include a commit to remove test_bitcoin_main.cpp. That file may or may not overwrite globals in the link stage depending on the link order. This is confusing and useless anyway: The unit tests should never std::exit in the middle of the run (especially with success as exit code), since it will skip all test modules afterward.

Also include a commit to remove some unused forward declarations and move the main_tests to validation_tests, since main was long ago split into net_processing and validation.

Concept ACK

Nice cleanup: -242 lines of code

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #15382 ([util] add runCommandParseJSON by Sjors)
• #14464 (refactor: make checkqueue manage the threads by itself (also removed some boost dependencies) by ken2812221)
• #10102 ([experimental] Multiprocess bitcoin by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

utACK faa9b88199c382c04e3aaa2b63479f9066ce8f9c. Looks good, bunch of nice simple make & test cleanups.