Test bootstrap signing flag validation

Covers the validation matrix of the new --gpg-* / --ssh-signing-*
surface: mutual exclusion (across GPG/SSH groups and within the SSH
group between --ssh-signing-key-file and --ssh-signing-reuse-private-
key), alias resolution between --ssh-signing-password and
--ssh-signing-passphrase, the dependency checks (--ssh-signing-
password requires --ssh-signing-key-file; --ssh-signing-reuse-
private-key requires --private-key-file), and pre-flight key-parse
failures (malformed PEM, encrypted SSH key without passphrase, GPG
ring with wrong passphrase). Test keys are checked in so the test
does not depend on local ssh-keygen or gpg invocations at run time.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

cmd/flux/testdata/bootstrap/ed25519.private

@@ -0,0 +1,7 @@
+-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACAWDldtCFdSMXIV1vLwXvRwk4eEmSoDCpxNkcbNph3dCAAAAIjjSDmx40g5
+sQAAAAtzc2gtZWQyNTUxOQAAACAWDldtCFdSMXIV1vLwXvRwk4eEmSoDCpxNkcbNph3dCA
+AAAEAGpzSFuLkCNDD49+tysxSFFwdOsRnDj67vDT9bfwoSDhYOV20IV1IxchXW8vBe9HCT
+h4SZKgMKnE2Rxs2mHd0IAAAABHRlc3QB
+-----END OPENSSH PRIVATE KEY-----