tests: introduce method to update reference data in isis_tilfa_topo1

The isis_tilfa_topo1 topotest is comprehensive and contains a large
amount of reference data. One problem is that, when changes occur,
updating this reference data can be difficult.

To address this problem, this commit introduces a method to
automatically regenerate the reference data by setting the `REGEN_DATA`
environment variable.

Usage:
$ REGEN_DATA=true python3 ./test_isis_tilfa_topo1.py

When `REGEN_DATA` is set, the topotest regenerates reference data
from the current run instead of comparing against existing reference
data. Note that regenerated data must be manually verified for
correctness.

This commit also simplifies the reference data by replacing all diff
files with complete JSON snapshots.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

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