selftests/vsock: identify and execute tests that can re-use VM

selftests/vsock: identify and execute tests that can re-use VM

In preparation for future patches that introduce tests that cannot
re-use the same VM, add functions to identify those that *can* re-use a
VM.

By continuing to re-use the same VM for these tests we can save time by
avoiding the delay of booting a VM for every test.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20251108-vsock-selftests-fixes-and-improvements-v4-8-d5e8d6c87289@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>