[perf-cleanup] remove redundant tests and update mcast polling (#9547)

This removes some test redundancy in the multicast suite and tries to make convergence faster for integration RPW testing.

I was spurred on to tackle this by way of #9536 and #9758. While this doesn't fully restore pre-multicast-introduced test times (likely due to more complete end-to-end network testing), it does reduce tail-end outliers when I run the `hist` tool against this branch.

Author: zeeshanlakhani

clients/sled-agent-client/src/lib.rs

@@ -359,6 +359,9 @@ impl ApiVersionHeader for reqwest::RequestBuilder {
 pub trait TestInterfaces {
     async fn vmm_single_step(&self, id: PropolisUuid);
     async fn vmm_finish_transition(&self, id: PropolisUuid);
+    /// Essentially like `vmm_finish_transition`, but returns an error instead
+    /// of panicking if the request fails. Useful when the VMM may have been
+    /// removed.
     async fn try_vmm_finish_transition(
         &self,
         id: PropolisUuid,