fn: fix flaky test with a benchmark

[Abdulkbk]

Fixes #9578

Change Description

As mentioned in the comment:

The test TestPropForEachConcOutperformsMapWhenExpensive compares the speed of Map and ForEachConc functions when executing an expensive operation on a slice. In this case, the expensive operation is time.Sleep(time.Millisecond).
Since the Map function operates sequentially, we expect ForEachConc to always complete first because it runs concurrently.
However ForEachConc has some overheads including creating and managing goroutines, semaphore acquisition & release, and waitgroup operations (which I think are non-deterministic).

The non-deterministic nature of ForEachConc, along with its overhead, can cause the Map function to sometimes finish executing before it, which results in CI failures.

This PR replaces the flaky test, comparing two functions, Map and ForEachConc, under expensive operations, with a benchmark.

Steps to Test

cd fn && go test -bench=BenchmarkMapVsForEachConc

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