test: do not use uninitialized memory in common flags check

Only use the amount of data that was actually read from the test file.
Otherwise, there is a small risk of getting false positives, and
generally reading uninitialized memory makes using automated
memory error detection tools harder.

PR-URL: #25475
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

test/common/index.js

@@ -57,9 +57,9 @@ if (process.argv.length === 2 &&
   const bytesToRead = 1500;
   const buffer = Buffer.allocUnsafe(bytesToRead);
   const fd = fs.openSync(module.parent.filename, 'r');
-  fs.readSync(fd, buffer, 0, bytesToRead);
+  const bytesRead = fs.readSync(fd, buffer, 0, bytesToRead);
   fs.closeSync(fd);
-  const source = buffer.toString();
+  const source = buffer.toString('utf8', 0, bytesRead);
 
   const flagStart = source.indexOf('// Flags: --') + 10;
   if (flagStart !== 9) {