pk_write test cases with short/long private key

Add pk_write test cases where the ASN.1 INTEGER encoding of the
private value would not have the mandatory size for the OCTET STRING
that contains the value.

ec_256_long_prv.pem is a random secp256r1 private key, selected so
that the private value is >= 2^255, i.e. the top bit of the first byte
is set (which would cause the INTEGER encoding to have an extra
leading 0 byte).

ec_521_short_prv.pem is a random secp521r1 private key, selected so
that the private value is < 2^519, i.e. the first byte is 0 and the
top bit of the second byte is 0 (which would cause the INTEGER
encoding to have one less 0 byte at the start).
diff --git a/tests/suites/test_suite_pkwrite.data b/tests/suites/test_suite_pkwrite.data
index c8ff177..e0101cc 100644
--- a/tests/suites/test_suite_pkwrite.data
+++ b/tests/suites/test_suite_pkwrite.data
@@ -30,10 +30,18 @@
 depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP192R1_ENABLED
 pk_write_key_check:"data_files/ec_prv.sec1.pem"
 
+Private key write check EC 256 bits (top bit set)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+pk_write_key_check:"data_files/ec_256_long_prv.pem"
+
 Private key write check EC 521 bits
 depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
 pk_write_key_check:"data_files/ec_521_prv.pem"
 
+Private key write check EC 521 bits (top byte is 0)
+depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_SECP521R1_ENABLED
+pk_write_key_check:"data_files/ec_521_short_prv.pem"
+
 Private key write check EC Brainpool 512 bits
 depends_on:MBEDTLS_ECP_C:MBEDTLS_BASE64_C:MBEDTLS_ECP_DP_BP512R1_ENABLED
 pk_write_key_check:"data_files/ec_bp512_prv.pem"