ci: Include test files filtering in Knapsack allocation

What does this MR do and why?

The goal is to optimize the distribution of the rspec * predictive jobs.

Let's imagine we have this Knapsack report of testfile -> duration:

Test file	Duration
testfile1	10
testfile2	20
testfile3	30
testfile4	40
testfile5	50

And based on the changes, detect-tests decides that we only need to run testfile1, testfile3, testfile5.

Let's say we parallelize the rspec * predictive jobs on two nodes.

Before

Before this change, the following would happen:

• First, Knapsack would generate a list of tests to be run on node1 and node2 without taking in account the list of test files that will actually be run. Based on the test files duration, the distribution would be:

Node	Test files	Total expected duration
node1	testfile5, testfile3	50 + 30 = 80
node2	testfile4, testfile2, testfile1	40 + 20 + 10 = 70

Then, our ParallelRSpecRunner wrapper would filter the list of tests further, so that the end result would be as follows:

Node	Test files	Total expected duration
node1	testfile5, testfile3	50 + 30 = 80
node2	testfile1	10

We can see that the distribution isn't balanced well.

After

With this change, the following would happen:

• First, Knapsack would generate a list of tests to be run on node1 and node2 with taking in account the list of test files that will actually be run. Based on the test files duration, the distribution would be:

Node	Test files	Total expected duration
node1	testfile5	50
node2	testfile3 testfile1	30 + 10 = 40

Then, our ParallelRSpecRunner wrapper doesn't have to filter the list of tests further since Knapsack already did the filtering.

We can see that the distribution is balanced way better now.

Next steps

As a further optimization, we should parallelize dynamically based on the number of test files to run, similarly to how we do with the rspec foss-impact child pipeline.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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