test: Fix shared random unit test for big endian

Copying the integer 42 in a char buffer has a different representation
depending on the endianess of the system thus that unit test was failing on
big endian system.

This commit introduces a python script, like the one we have for SRV, that
computes a COMMIT/REVEAL from scratch so we can use it as a test vector for
our encoding unit tests.

With this, we use a random value of bytes instead of a number fixing the
endianess issue and making the whole test case more solid with an external
tool that builds the COMMIT and REVEAL according to the spec.

Fixes #19977

Signed-off-by: David Goulet <dgoulet@torproject.org>