Prior to this change there were two problems with our fuzzing for oss-fuzz:
1. There was an issue if the fuzzing spanned two files (mingled with the testing).
2. The fuzzing needs to be part of the implementation package (no _test packages).
This change fixes that by moving all package fuzzing into a common `fuzz_test.go` within the package.
Updates https://github.com/tailscale/corp/issues/46608
Change-Id: I0b95edcd0df946f723eea32f575c679214c0b202
Signed-off-by: Mike Jensen <mikej@tailscale.com>