Use random weights in the unit test for multi-src flux conserve

[mcara]

Improves unit test added in #177 by using random weights for input image pixels compared to constant weight set to one in the original test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.47%. Comparing base (00f57f6) to head (a6e41db).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #178   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files           3        3           
  Lines         238      238           
=======================================
  Hits          232      232           
  Misses          6        6           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.