tstest/membudget: add live-heap budget regression tests

Static size checks (iossize) catch binary dirty-page growth but nothing
covered runtime heap cost, which is what actually consumes the iOS
Network Extension's 50 MiB jetsam budget. Bring up a tsnet backend
(with the full condregister feature set, matching shipping clients)
against an in-process testcontrol server and assert live post-GC heap
budgets for (a) backend startup with zero peers and (b) marginal cost
per netmap peer.

The startup test measures 1.3 MiB today and fails loudly on the
conn25 flow-table pre-allocation regression (17 MiB) that jetsam-killed
the iOS extension on large tailnets.

Budgets are deliberately generous (6-12x current measurements) to stay
flake-free while still catching the multi-MiB regressions that matter
for mobile.

A new debugknob enables us to constrain the GSO/GRO batch size to 1 for
these tests so as to avoid the memory allocation associated with those
buffers, which are a known issue with their own work stream.

Updates tailscale/corp#46408
Updates tailscale/corp#18514

Signed-off-by: James Tucker <james@tailscale.com>