Clarify the error message for an ECDSA power-on test failure.

We want to clarify that this isn't the PWCT that FIPS generally means,
but rather the power-on self-test. Since ECDSA is non-deterministic, we
have to implement that power-on self-test as a PWCT, but we have a
different flag to break that actual PWCT.

Change-Id: I3e27c6a6b0483a6c04e764d6af8a4a863e0b8b77
Reviewed-on: https://boringssl-review.googlesource.com/16765
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/bcm.c

@@ -604,7 +604,7 @@ static void BORINGSSL_bcm_power_on_self_test(void) {
   if (sig == NULL ||
       !ECDSA_do_verify(kPlaintextSHA256, sizeof(kPlaintextSHA256), sig,
                        ec_key)) {
-    printf("ECDSA Sign/Verify PWCT failed.\n");
+    printf("ECDSA Sign/Verify power-on PWCT failed.\n");
     goto err;
   }