Add test for EC keys with all curves.

(Made possible by the OID fix.)
diff --git a/tests/data_files/ec_256_pub.pem b/tests/data_files/ec_256_pub.pem
new file mode 100644
index 0000000..701da02
--- /dev/null
+++ b/tests/data_files/ec_256_pub.pem
@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEd3Jlb4FLOZJ51eHxeB+sbwmaPFyh
+sONTUYNLCLZeC1clkM2vj3aTYbzzSs/BHl4HToQmvd4Evm5lOUVElhfeRQ==
+-----END PUBLIC KEY-----