Add test for EC keys with all curves.

(Made possible by the OID fix.)
diff --git a/tests/data_files/ec_224_prv.pem b/tests/data_files/ec_224_prv.pem
new file mode 100644
index 0000000..ebb83a0
--- /dev/null
+++ b/tests/data_files/ec_224_prv.pem
@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MGgCAQEEHGhJ+X0QZvaZd1ljfH44mUZM7j7HrJcGU6C+B0KgBwYFK4EEACGhPAM6
+AAQWk6KQ9/C1cf4rQdXYSwEydjH0qGD5lfozLAl/VBkrsQ8AET8q/7E8GiTORJFF
+calUQK4BSgDL9w==
+-----END EC PRIVATE KEY-----