test: verify input flags

This makes sure all required flags are passed through to the test.
If that's not the case an error is thrown to inform the user what
flag is missing.

test/common/index.js

@@ -45,6 +45,44 @@ const isMainThread = (() => {
   }
 })();
 
+// Check for flags. Skip this for workers (both, the `cluster` module and
+// `worker_threads`) and child processes.
+if (process.argv.length === 2 &&
+    isMainThread &&
+    module.parent &&
+    require('cluster').isMaster) {
+  // The copyright notice is relatively big and the flags could come afterwards.
+  const bytesToRead = 1500;
+  const buffer = Buffer.allocUnsafe(bytesToRead);
+  const fd = fs.openSync(module.parent.filename, 'r');
+  fs.readSync(fd, buffer, 0, bytesToRead);
+  fs.closeSync(fd);
+  const source = buffer.toString();
+
+  const flagStart = source.indexOf('// Flags: --') + 10;
+  if (flagStart !== 9) {
+    let flagEnd = source.indexOf('\n', flagStart);
+    // Normalize different EOL.
+    if (source[flagEnd - 1] === '\r') {
+      flagEnd--;
+    }
+    const flags = source
+      .substring(flagStart, flagEnd)
+      .replace(/_/g, '-')
+      .split(' ');
+    const args = process.execArgv.map((arg) => arg.replace(/_/g, '-'));
+    for (const flag of flags) {
+      if (!args.includes(flag) &&
+          // If the binary is build without `intl` the inspect option is
+          // invalid. The test itself should handle this case.
+          (process.config.variables.v8_enable_inspector !== 0 ||
+            !flag.startsWith('--inspect'))) {
+        throw new Error(`Test has to be started with the flag: '${flag}'`);
+      }
+    }
+  }
+}
+
 const isWindows = process.platform === 'win32';
 const isAIX = process.platform === 'aix';
 const isLinuxPPCBE = (process.platform === 'linux') &&