Add success case for pem_read testing

Currently all cases were negative, so the block that exercised
mbedtls_pem_get_der() would never be reached.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/suites/test_suite_pem.data b/tests/suites/test_suite_pem.data
index 77546c5..a42fe36 100644
--- a/tests/suites/test_suite_pem.data
+++ b/tests/suites/test_suite_pem.data
@@ -16,6 +16,9 @@
 PEM write (exactly two lines + 1)
 mbedtls_pem_write_buffer:"-----START TEST-----\n":"-----END TEST-----\n":"000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F00":"-----START TEST-----\nAAECAwQFBgcICQoLDA0ODwABAgMEBQYHCAkKCwwNDg8AAQIDBAUGBwgJCgsMDQ4P\nAAECAwQFBgcICQoLDA0ODwABAgMEBQYHCAkKCwwNDg8AAQIDBAUGBwgJCgsMDQ4P\nAA==\n-----END TEST-----\n"
 
+PEM read (unencrypted, valid)
+mbedtls_pem_read_buffer:"^":"$":"^\nTWJlZCBUTFM=\n$":"":0
+
 PEM read (DES-EDE3-CBC + invalid iv)
 mbedtls_pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,00$":"pwd":MBEDTLS_ERR_PEM_INVALID_ENC_IV