test(presubmit): prevent panic in TestImportOrdering on split import (#7540)

Fix a panic in presubmit test when import statements are split into
>3 logical blocks (e.g., std, coredns, then third party in multiple
blocks). The computed block index could exceed the fixed array
bounds.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

test/presubmit_test.go

@@ -238,14 +238,20 @@ func (w *testImportOrderingWalker) walk(path string, info os.FileInfo, _ error)
 	blocks := [3][]*ast.ImportSpec{}
 	prevpos := 0
 	bl := 0
+	reportedTooManyBlocks := false
 	for _, im := range f.Imports {
 		line := fs.Position(im.Path.Pos()).Line
 		if line-prevpos > 1 && prevpos > 0 {
 			bl++
 		}
 		if bl > 2 {
+			if !reportedTooManyBlocks {
 				absPath, _ := filepath.Abs(path)
 				w.Errors = append(w.Errors, fmt.Errorf("more than %d import blocks in %q", bl, absPath))
+				reportedTooManyBlocks = true
+			}
+			// Clamp to last valid block index to avoid out-of-bounds access
+			bl = 2
 		}
 		blocks[bl] = append(blocks[bl], im)
 		prevpos = line