test_esplora_sync is flaky

[TheBlueMatt]

CI said this on an unrelated PR:

running 1 test
test test_esplora_syncs ... FAILED

failures:
error: test failed, to rerun pass `--test integration_tests`

---- test_esplora_syncs stdout ----
thread 'test_esplora_syncs' panicked at 'called `Result::unwrap()` on an `Err` value: JSON(Error("expected value", line: 1, column: 1))', lightning-transaction-sync/tests/integration_tests.rs:63:70
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_esplora_syncs

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 1.55s

[tnull]

Ugh, this is weird. I'll put some changes up that should make it more robust by giving the bitcoind/electrsd instances more leeway.

[TheBlueMatt]

No rush, its probably just CI being garbage and impossibly slow, as usual.