workflows: enable Mbed TLS based EC backend tests

Enable sig-ecdsa-mbedtls and enc-ec256-mbedtls simulator tests.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.github/workflows/sim.yaml b/.github/workflows/sim.yaml
index c866715..63a8872 100644
--- a/.github/workflows/sim.yaml
+++ b/.github/workflows/sim.yaml
@@ -26,10 +26,7 @@
         - "sig-ecdsa-mbedtls enc-kw validate-primary-slot"
         - "sig-rsa validate-primary-slot overwrite-only large-write"
         - "sig-ecdsa enc-ec256 validate-primary-slot"
-        # ecdsa-mbedtls and enc-ec256 are not currently supported
-        # together, as the ec256 code requires only one backend be
-        # active.
-        # - "sig-ecdsa-mbedtls enc-ec256 validate-primary-slot"
+        - "sig-ecdsa-mbedtls enc-ec256-mbedtls validate-primary-slot"
         - "sig-rsa validate-primary-slot overwrite-only downgrade-prevention"
     runs-on: ubuntu-latest
     env: