Use DER encoded keys with tinycrypt

PEM and BASE64 aren't used with baremetal config by default
and using DER encoded key enables the test without enabling PEM
and BASE64.
diff --git a/tests/data_files/ec_256_pub.der b/tests/data_files/ec_256_pub.der
new file mode 100644
index 0000000..2ba2595
--- /dev/null
+++ b/tests/data_files/ec_256_pub.der
Binary files differ
diff --git a/tests/suites/test_suite_pk.data b/tests/suites/test_suite_pk.data
index 5bdbea0..c8f4d70 100644
--- a/tests/suites/test_suite_pk.data
+++ b/tests/suites/test_suite_pk.data
@@ -154,7 +154,7 @@
 
 Check pair #2 (EC, bad, TinyCrypt)
 depends_on:MBEDTLS_USE_TINYCRYPT
-mbedtls_pk_check_pair:"data_files/ec_256_pub.pem":"data_files/server5.key":MBEDTLS_ERR_PK_BAD_INPUT_DATA
+mbedtls_pk_check_pair:"data_files/ec_256_pub.der":"data_files/server5.key.der":MBEDTLS_ERR_PK_BAD_INPUT_DATA
 
 Check pair #3 (RSA, OK)
 depends_on:MBEDTLS_RSA_C:MBEDTLS_PKCS1_V15