fix: traffic-split weight distribution and add e2e tests

[Revolyssup]

This PR solves two bugs and adds corresponding E2E tests:

1. backendErr is currently a shared variable due to which even if one backend is invalid, no backend is set causing 0 nodes to be set on upstream even when there are valid nodes. A test case confirming the fix for this has been added. The test valid backend is set even if other backend is invalid confirms this fix.
2. The weight from backend was not set in the upstream label which is used to set weight on weightedUpstreams due to which all nodes always get 100 as weight. The tests 2:1 traffic split test and zero-weight test confirm this fix.

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[ronething]

lgtm