Tapify libtestutil a bit better

This includes better signals of skips and subtests according to TAP 12,
and flushing stdout and stderr at the end of every test function to
make sure we get the output in good order.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3309)

test/testutil/driver.c

@@ -138,13 +138,23 @@ int run_tests(const char *test_prog_name)
     char *verdict = NULL;
     int i, j;
 
-    helper_printf_stdout("%*s%d..%d\n", level, "", 1, num_tests);
+    if (num_tests < 1)
+        helper_printf_stdout("%*s1..0 # Skipped: %s\n", level, "",
+                             test_prog_name);
+    else if (level > 0)
+        helper_printf_stdout("%*s1..%d # Subtest: %s\n", level, "", num_tests,
+                             test_prog_name);
+    else
+        helper_printf_stdout("%*s1..%d\n", level, "", num_tests);
     test_flush_stdout();
 
     for (i = 0; i != num_tests; ++i) {
         if (all_tests[i].num == -1) {
             int ret = all_tests[i].test_fn();
 
+            test_flush_stdout();
+            test_flush_stderr();
+
             verdict = "ok";
             if (!ret) {
                 verdict = "not ok";
@@ -169,6 +179,9 @@ int run_tests(const char *test_prog_name)
             for (j = 0; j < all_tests[i].num; j++) {
                 int ret = all_tests[i].param_test_fn(j);
 
+                test_flush_stdout();
+                test_flush_stderr();
+
                 if (!ret)
                     ++num_failed_inner;