Fix bug in SwitchTraffic that wasn't respecting --dry_run for rea…

…donly and replica tablets during a resharding event (vitessio#12992)

* use switcher struct when switching shard reads during a reshard event

Signed-off-by: austenLacy <[email protected]>

* Create failing test for bug reported in vitessio#12992, where a TrafficSwitch dry run for reads during resharding tries to actually switch reads and fails

Signed-off-by: Rohit Nayak <[email protected]>

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Signed-off-by: austenLacy <[email protected]>
Signed-off-by: Rohit Nayak <[email protected]>
Co-authored-by: Rohit Nayak <[email protected]>
Signed-off-by: 'Stanislav Maksimov' <[email protected]>