tests: fix lfa test (28)
tests: fix lfa test (28)

It turns out that LFA cannot handle any failure, even if LFA
alternates are possible.
Example in the context of test 28:
- AD goes down --> A decides to reach D through B, and D decides
  to reach A through B. That's fine.
- AB goes down --> A decides to reach D through C, but D will
  still route through B, that will just drop the packets. D
  could successfully route through C, but it does not have the
  necessary information to understand that.
This does not look like to an LFA bug, but rather a limitation.

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