Bug Report: `fileNameFromPosition` in `plannedreparent` unit tests fails on faster hardware

[timvaillancourt]

Overview of the Issue

We're noticing that TestFullStatus (in go/test/endtoend/reparent/plannedreparent) fails on all Vitess versions >= v14 when too many binlog entries are created due to faster test runner hardware. This longer position breaks the assumptions in the test-helper func fileNameFromPosition

Thanks @deepthi for helping me find this problem 🙇

Reproduction Steps

This bug can be reproduced in this Go Playground, which has the same logic as fileNameFromPosition: https://go.dev/play/p/ck5jEMcZu_V

Binary Version

At least v14 and higher

Operating System and Environment details

Any environment due to unit tests

Log Fragments

reparent_test.go:168: 
        	Error Trace:	/home/runner/work/vitess/vitess/go/test/endtoend/reparent/newfeaturetest/reparent_test.go:168
        	Error:      	Not equal: 
        	            	expected: "FilePos/vt-0000000101-bin.000001"
        	            	actual  : "FilePos/vt-0000000101-bin.000001:"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-FilePos/vt-0000000101-bin.000001
        	            	+FilePos/vt-0000000101-bin.000001:
        	Test:       	TestFullStatus
--- FAIL: TestFullStatus (21.13s)