This test restarts l2 twice. Each time, l1 is reconnecting, and backs
off. If the test is slow enough, the backoff gets extreme:
```
2026-02-19T02:13:03.7669982Z lightningd-1 2026-02-19T01:50:56.541Z DEBUG 033845802d25b4e074ccfd7cd8b339a41dc75bf9978a034800444b51d42b07799a-lightningd: peer_disconnected
2026-02-19T02:13:03.7670444Z lightningd-1 2026-02-19T01:50:56.547Z DEBUG 033845802d25b4e074ccfd7cd8b339a41dc75bf9978a034800444b51d42b07799a-connectd: Will try reconnect in 256 seconds
```
This isn't a bug! The backoff caps at 300 seconds, and only gets
reset if we remain connected for that long.
A manual reconnect here not only fixes the flake, but make the test
much faster, by not *doubling* the time for slow tests as shown on my
laptop (the final test using `taskset -c 1`):
Normal Valgrind Valgrind, 1 CPU
Before: 22sec 124sec 230sec
After: 18sec 102sec 191sec
These are from a single run: it could be much more in the worst case.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>