silence system utils tests in emscripten python.sh

[Lukasdoe]

When running the generated python.sh an a system without grealpath (like Linux), which prints to stderr and pollutes the console with an unnecessary error message:

which: no grealpath in (<$PATH>)
Python 3.14.0a7+ 
Type "help", "copyright", "credits" or "license" for more information.
>>> 

where <$PATH> is the system path which may span multiple lines.

This PR redirects all output to /dev/null, since we're only interested in the return code anyways.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[freakboy3742]

Looks like a good improvement to me - Thanks for the fix!