CI: Add some randomness to the ports used in VReplication e2e tests

[mattlord]

Description

The vreplication_migrate workflow which runs the TestMigrateUnsharded and TestMigrateSharded tests has been a little flaky. The failures that I've seen are all related to port re-use. For example, from the most recent non-PR changes related failure:

2025-02-05T19:46:04.6078463Z F0205 19:45:45.485443   37959 grpc_server.go:305] Cannot listen on port 28192 for gRPC: listen tcp 127.0.0.1:28192: bind: address already in use
2025-02-05T19:46:04.6078885Z 
2025-02-05T19:46:04.6079010Z     cluster_test.go:661: 
2025-02-05T19:46:04.6079670Z         	Error Trace:	/home/runner/work/vitess/vitess/go/test/endtoend/vreplication/cluster_test.go:661
2025-02-05T19:46:04.6080863Z         	            				/home/runner/work/vitess/vitess/go/test/endtoend/vreplication/cluster_test.go:472
2025-02-05T19:46:04.6082094Z         	            				/home/runner/work/vitess/vitess/go/test/endtoend/vreplication/migrate_test.go:260
2025-02-05T19:46:04.6083233Z         	            				/home/runner/work/vitess/vitess/go/test/endtoend/vreplication/migrate_test.go:229
2025-02-05T19:46:04.6083746Z         	Error:      	Received unexpected error:
2025-02-05T19:46:04.6084837Z         	            	process 'extcell1-1201' timed out after 10s (err: process 'extcell1-1201' exited prematurely (err: exit status 1))
2025-02-05T19:46:04.6085392Z         	Test:       	TestMigrateSharded

What makes this workflow and its tests somewhat unique is that each test creates two Vitess clusters (each with its own etcd server). So there was a fair amount of port re-use. So if there was any parallelism on the host or we failed to kill one of the test processes at the end we'd encounter a failure due to trying to bind to a port that's already in use.

This PR tries to address (or at least mitigate) the issue by adding some randomness to the ports used.

I re-ran the workflow 20 times here to confirm that we saw no failures, and we did not: https://github.com/vitessio/vitess/actions/runs/13186110424/job/36808474378?pr=17712

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.95%. Comparing base (10a6916) to head (2e6f38a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17712   +/-   ##
=======================================
  Coverage   67.94%   67.95%           
=======================================
  Files        1586     1586           
  Lines      255173   255173           
=======================================
+ Hits       173389   173402   +13     
+ Misses      81784    81771   -13     

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