Read and write X25519 and X448 private keys

Signed-off-by: Jethro Beekman <jethro@fortanix.com>
Co-authored-by: Gijs Kwakkel <gijs.kwakkel@fortanix.com>
Signed-off-by: Gijs Kwakkel <gijs.kwakkel@fortanix.com>
diff --git a/tests/data_files/ec_x448_prv.pem b/tests/data_files/ec_x448_prv.pem
new file mode 100644
index 0000000..7bca661
--- /dev/null
+++ b/tests/data_files/ec_x448_prv.pem
@@ -0,0 +1,4 @@
+-----BEGIN PRIVATE KEY-----
+MEYCAQAwBQYDK2VvBDoEOGTpCDYD9atLDMYwDnjdwUTkjO4ZMB/uacXKw+4iUiED
+oe50tXsIxi5REqWe2YOoL7eD7npOGRPt
+-----END PRIVATE KEY-----