test_select_only_expired_subkey

Test key is generated with:
```
#!/bin/sh
set -e
export GNUPGHOME="$PWD/gnupghome"
rm -fr  "$GNUPGHOME"
mkdir -p "$GNUPGHOME"
chmod 700 "$GNUPGHOME"

GPG="gpg --batch --quiet --faked-system-time 20070924T154812"
USERID='Alice <alice@example.org>'
$GPG --passphrase '' --quick-generate-key "$USERID" ed25519 default never
FINGERPRINT="$($GPG --list-secret-keys --with-colons | awk -F: '$1 == "fpr" { print $10 }')"
$GPG --passphrase '' --quick-add-key "$FINGERPRINT" cv25519 encr 1y
$GPG --armor --export "$FINGERPRINT" > alice.tpk.asc
```

src/pgp/pgp_tests.rs

@@ -505,3 +505,28 @@ FFoiBQJG99w8AhsMAAoJECcqAms0FFoirSMA/0gVP98sPFga+UhQ3uJxJw5bO2Rs
         expected_expiring_fingerprint
     );
 }
+
+/// Tests that key selection fails if there is only an expired subkey.
+#[test]
+fn test_select_only_expired_subkey() {
+    let alice_tpk_asc = "
+    -----BEGIN PGP PUBLIC KEY BLOCK-----
+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+=zWPq
+-----END PGP PUBLIC KEY BLOCK-----
+";
+
+    let alice_tpk = SignedPublicKey::from_asc(alice_tpk_asc).unwrap();
+    let now = pgp::types::Timestamp::now();
+
+    assert_eq!(select_pk_for_encryption(now.as_secs(), &alice_tpk), None);
+}