test: correct header length subtraction

In test-http-max-http-headers, a comment asks why we are subtracting 32
from the length of the invalid-length-by-1 headers instead of just 1.
Subtracting 1 seems to be correct and works.

PR-URL: https://github.com/nodejs/node/pull/30712


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>