Use a shared common body for all the tests: fork1..fork8, vfork1..vfork8.
Merge vforkdone1 and vforkdone2 into vfork* tests.
All the (v?)fork[1-8] tests cover:
- calling either fork(2) or vfork(2)
- tracking either enabled or disabled FORK, VFORK or VFORK_DONE
All the PTRACE_VFORK tests are marked as expected failure.
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