preludes: fix v -skip-unused -stats some_test.v

vlib/v/preludes/tests_with_stats.v

@@ -24,6 +24,7 @@ mut:
 
 // ///////////////////////////////////////////////////////////////////
 // Called at the start of the test program produced by `v -stats file_test.v`
+@[markused]
 fn start_testing(total_number_of_tests int, vfilename string) BenchedTests {
 	mut benched_tests_res := BenchedTests{
 		bench: benchmark.new_benchmark()
@@ -35,6 +36,7 @@ fn start_testing(total_number_of_tests int, vfilename string) BenchedTests {
 }
 
 // Called before each test_ function, defined in file_test.v
+@[markused]
 fn (mut b BenchedTests) testing_step_start(stepfunc string) {
 	b.step_func_name = stepfunc.replace('main.', '').replace('__', '.')
 	b.oks = test_runner.total_assert_passes
@@ -44,6 +46,7 @@ fn (mut b BenchedTests) testing_step_start(stepfunc string) {
 }
 
 // Called after each test_ function, defined in file_test.v
+@[markused]
 fn (mut b BenchedTests) testing_step_end() {
 	ok_diff := int(test_runner.total_assert_passes - b.oks)
 	fail_diff := int(test_runner.total_assert_fails - b.fails)
@@ -79,6 +82,7 @@ fn (b &BenchedTests) fn_name() string {
 }
 
 // Called at the end of the test program produced by `v -stats file_test.v`
+@[markused]
 fn (mut b BenchedTests) end_testing() {
 	b.bench.stop()
 	fname := os.file_name(b.test_suit_file)