Follow Aditya's suggestion to "embed" all needed testfiles (ssh keys and
askpass script) as bytes and write them to a tmp dir where needed:
https://github.com/gittuf/gittuf/pull/414#discussion_r1618647676
Removes previously added testutils.go
**Interesting discovery**
In order to sign with an "rsa" key, ssh-keygen seems to expect an
"rsa.pub" in the same directory. This is not the case for encrypted rsa,
no for plaintext or encrypted ecdsa or ed25519.
Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>
