fix(tests): disable MBedTLS in CI due to deadlock state in its code

I'm pretty sure the deadlock issue is unrelated to our code, as MBedTLS
hangs up while attempting to lock some variable related to one of its
RNGs.

My suspicion is that it has to do with the fact that we use multiple
MBedTLS contexts in the same process.

Author: pulsastrix

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
       fail-fast: false
       matrix:
         crate: [libcoap-sys, libcoap-rs]
-        dtls_backend: [openssl, gnutls, tinydtls, mbedtls]
+        dtls_backend: [openssl, gnutls, tinydtls]
     steps:
       - uses: actions/checkout@v3
         with: