Testing is very important, but adding additional code to accommodate makes the simulation even _less_ faithful, and more likely to miss real issues (or potentially even introduce real issues).  I do regression testing with a ASIC miner and the main network code (with checkpoints=0, of course). Testing with the actual production-time behavior is the gold standard and cannot be replaced with shortcutted version without compromise.  (FWIW, testnet which, in spite of its own stupid shortcuts, is somewhat closer to Bitcoin has test cases in the chain for adjustment extremes).

The only reason you were able to make this comment at all is because regtest exists, and the only reason regtest mode exists is because it was specifically created for the block tester harness that runs on externally hosted ci setup, e.g. its intended use. For a long time the harness applied a patch to change the behavior to make it computationally cheaper to test-- at the expense of making them less accurate and faithful--, but maintaining the patch externally took work.

Use of it has expanded since then--  I think today a somewhat different approach would make more sense for the regtest shortcutting and would result in a smaller divergence from the normal network behavior.  (e.g. when in testing mode, mask out the highest bits of the block hashes before the target check).  So quite the opposite, testing is important enough that one should actually be testing the actual Bitcoin network code and not a altcoinified mockup that makes testing easier, or to the extent a modified version for testability is used great care should be taken to minimize the number of differences (and  to not add risk to production code).  How you could extract "testing is not important" from my comments about a whole alternative network mode created specifically for testing is beyond me-- specifically given the amount of effort I put in previously in convincing you that agreement testing was essential.