tests: make the flaky array_access_optimisation_test.v failures on the macos CI, more easily diagnosable

vlib/v/tests/array_access_optimisation_test.v

@@ -39,7 +39,12 @@ fn test_array_optimisation() {
 	p.wait()
 	p.close()
 
-	assert stdout.contains('// THE END.')
+	ending := '// THE END.'
+	it_ends_properly := stdout.contains(ending)
+	if !it_ends_properly {
+		dump(stdout)
+	}
+	assert it_ends_properly
 
 	for line in stdout.split('\n') {
 		direct(line)