Add test for export keys functionality

Add test in `ssl-opts.sh` that the export keys callback
is actually called.
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 08d4be3..cef87bc 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -7939,6 +7939,18 @@
             -s "Extra-header:" \
             -c "Extra-header:"
 
+requires_config_enabled MBEDTLS_SSL_EXPORT_KEYS
+run_test    "export keys functionality" \
+            "$P_SRV eap_tls=1 debug_level=3" \
+            "$P_CLI eap_tls=1 debug_level=3" \
+            0 \
+            -s "exported maclen is " \
+            -s "exported keylen is " \
+            -s "exported ivlen is "  \
+            -c "exported maclen is " \
+            -c "exported keylen is " \
+            -c "exported ivlen is "
+
 # Final report
 
 echo "------------------------------------------------------------------------"