Add tests for offline sync with db boostraps

[avinassh]

We have a mock server

libsql/libsql/src/sync/test.rs

Lines 314 to 320 in 618505c

	struct MockServer {
	url: String,
	frame_count: Arc<AtomicU32>,
	connector: ConnectorService,
	return_error: Arc<AtomicBool>,
	request_count: Arc<AtomicU32>,
	}

but it does not support /export to test bootstrap. Lets update the mock to consider /export and also add more tests for offline sync which does bootstrap of db

[assadyousuf]

@avinassh Have a PR for this: #2152