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: https://github.com/nodejs/node/pull/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>