Add more tests for UDP socket splicing and enhance existing tests.

Reengineer the socket splicing regression tests:
- Move the tests from splice to new sosplice directory for consistent naming.
- Split the API tests and the TCP splicing tests into separate directori

Reengineer the socket splicing regression tests:
- Move the tests from splice to new sosplice directory for consistent naming.
- Split the API tests and the TCP splicing tests into separate directories.
- Create some tests for the upcoming UDP splicing.
- Tests can be run in obj directories now.
- The API tests can run both on a local and on a remote machine now.
- Fix the forking TCP tests which splice and read or write simultaneously.
- Bunch of little fixes for races in the tests.
- Deduplicate code, move checks into common functions.

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