test: add integration tests

test/integration/testdata/pem/README.md

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+To generate a valid certificate and private key use the following command:
+
+```bash
+# In OpenSSL ≥ 1.1.1
+openssl req -x509 -newkey rsa:4096 -sha256 -days 9999 -nodes \
+  -keyout key.pem -out cert.pem -subj "/CN=localhost" \
+  -addext "subjectAltName=DNS:localhost,IP:127.0.0.1,IP:::1"
+```
+
+To check a certificate use the following command:
+
+```bash
+openssl x509 -in cert.pem -text
+```