Enable keyfromlabel and enginepkcs11 systemtests with pkcs11-provider

- Simplify configuration management by deducing SoftHSM module path
  from openssl config

- Determine the engine flag (-E) value from openssl config

- Drop unused/unneeded environment variables

- Run pkcs11-provider tests on Debian "sid" ossl3 flavor

bin/tests/system/enginepkcs11/prereq.sh

@@ -13,9 +13,20 @@
 
 . ../conf.sh
 
-if [ -n "${SOFTHSM2_MODULE}" ] && command -v softhsm2-util >/dev/null; then
-	exit 0
-fi
+[ -n "${SOFTHSM2_CONF}" ] || {
+    echo_i "skip: softhsm2 configuration not available"
+    exit 255
+}
 
-echo_i "skip: softhsm2-util not available"
-exit 255
+parse_openssl_config
+[ -f "$SOFTHSM2_MODULE" ] || {
+    echo_i "skip: softhsm2 module not available"
+    exit 1
+}
+
+for _bin in softhsm2-util pkcs11-tool; do
+    command -v "$_bin" >/dev/null || {
+        echo_i "skip: $_bin not available"
+        exit 1
+    }
+done