test: correct iteration over pids in SUT (#2693)

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## Overview

NOTE: the fix was actually included in the e2e refactor, this PR is just
adding some extra tests to cover it.

I found a small bug in the `SystemUnderTest` type in which iteration was
happening over indices instead of process IDs, so when Shutdown was
called, it was not actually finding the process.

The fix was

```diff
- for pid := range pids {
+ for _, pid := range pids {
	p, err := os.FindProcess(pid)
	if err != nil {
		continue
	}
	if !yield(p) {
		break
	}
}
```

in `iterProcessesByCmd`


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Author: chatton

test/e2e/sut_helper_test.go

@@ -0,0 +1,52 @@
+//go:build e2e
+
+package e2e
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestIterProcessesByCmd tests that iterProcessesByCmd correctly iterates over actual PIDs.
+func TestIterProcessesByCmd(t *testing.T) {
+	sut := NewSystemUnderTest(t)
+
+	// simulate some processes being tracked
+	testPIDs := []int{12345, 67890, 99999}
+	cmdName := "testcmd"
+
+	// manually add PIDs to the tracking maps.
+	for _, pid := range testPIDs {
+		sut.pids[pid] = struct{}{}
+		sut.cmdToPids[cmdName] = append(sut.cmdToPids[cmdName], pid)
+	}
+
+	var iteratedPIDs []int
+	for process := range sut.iterProcessesByCmd(cmdName) {
+		iteratedPIDs = append(iteratedPIDs, process.Pid)
+	}
+
+	// The iterator should return the actual PIDs
+	require.ElementsMatch(t, testPIDs, iteratedPIDs,
+		"iterProcessesByCmd should iterate over actual PIDs")
+}
+
+// TestShutdownByCmd tests that ShutdownByCmd can properly find and signal processes.
+// This test verifies that the fix allows the shutdown mechanism to work correctly.
+func TestShutdownByCmd(t *testing.T) {
+	sut := NewSystemUnderTest(t)
+
+	sut.ExecCmd("sleep")
+
+	require.Eventually(t, func() bool {
+		return sut.HasProcess("sleep")
+	}, time.Millisecond*100, time.Millisecond*10, "process should be tracked after starting")
+
+	sut.ShutdownByCmd("sleep")
+
+	require.Eventually(t, func() bool {
+		return !sut.HasProcess("sleep")
+	}, time.Millisecond*100, time.Millisecond*10, "process should be removed from tracking after shutdown")
+}