Add tests for x509parse_key_ec()

Test files were generated as follows:

openssl ecparam -name prime192v1 -genkey > key.pem

openssl ec -in key.pem -pubout -outform PEM > pub.pem
openssl ec -in key.pem -pubout -outform DER > pub.der

openssl ec -in key.pem -outform pem > prv.sec1.pem
openssl ec -in key.pem -outform der > prv.sec1.der
openssl ec -in key.pem -des -passout pass:polar -outform pem > prv.sec1.pw.pem

openssl pkcs8 -topk8 -in key.pem -nocrypt -outform pem > prv.pk8.pem
openssl pkcs8 -topk8 -in key.pem -nocrypt -outform der > prv.pk8.der
openssl pkcs8 -topk8 -in key.pem -passout pass:polar -outform der \
    > prv.pk8.pw.der
openssl pkcs8 -topk8 -in key.pem -passout pass:polar -outform pem \
    > prv.pk8.pw.pem

tests/data_files/ec_prv.pk8.pem

@@ -0,0 +1,5 @@
+-----BEGIN PRIVATE KEY-----
+MG8CAQAwEwYHKoZIzj0CAQYIKoZIzj0DAQEEVTBTAgEBBBgzjoaogeI49Um9bwVT
+SUtz49YRMP3GyW2hNAMyAARRdbzfMKNw851Tk+YScojYAWe19LS3dsZ098bzVLfS
+JAYsH2hUtaevD+V46vJY8Cc=
+-----END PRIVATE KEY-----

tests/data_files/ec_prv.pk8.pw.pem

@@ -0,0 +1,6 @@
+-----BEGIN ENCRYPTED PRIVATE KEY-----
+MIGXMBsGCSqGSIb3DQEFAzAOBAiO5a+OuKjOdwICCAAEeGEBv8oxHZy3GvR28QHJ
+Xq1Fq/UZrWklH6RI8VTx4WS6UK88PzJSmLMxO3pl4L/CUNfJ4RI9+YHFRZyf8Tcr
+5di4xMQaEqh08PXwyErqOerBGc9DsFpcpFAsn6Sa9KLG5aritGrSkPyDACloHyKR
+lJyTE6fTVwG5xQ==
+-----END ENCRYPTED PRIVATE KEY-----

tests/data_files/ec_prv.sec1.pem

@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MF8CAQEEGDOOhqiB4jj1Sb1vBVNJS3Pj1hEw/cbJbaAKBggqhkjOPQMBAaE0AzIA
+BFF1vN8wo3DznVOT5hJyiNgBZ7X0tLd2xnT3xvNUt9IkBiwfaFS1p68P5Xjq8ljw
+Jw==
+-----END EC PRIVATE KEY-----

tests/data_files/ec_prv.sec1.pw.pem

@@ -0,0 +1,8 @@
+-----BEGIN EC PRIVATE KEY-----
+Proc-Type: 4,ENCRYPTED
+DEK-Info: DES-CBC,AA94892A169FA426
+
+gSkFuUENNke5MvkWHc11/w1NQWBxaIxGT+d5oRcqs44D3tltVOwtdnYexoD9uSIL
+wMFFRLL6I5ii1Naa38nPOMaa7kLU2J3jY8SeIH1rQ43X6tlpv9WFGqDn/m6X7oKo
+RMMfGdicPZg=
+-----END EC PRIVATE KEY-----