tests: constify command-line arguments

For libtests, tunits, units.

Also:
- lib3033: tidy up headers.
- lib/netrc: constify an arg in `Curl_parsenetrc()`.

Closes #18076

tests/unit/unit1399.c

@@ -71,7 +71,7 @@ static void expect_timer_seconds(struct Curl_easy *data, int seconds)
  * E.g., if t_starttransfer took 2 seconds initially and took another 1
  * second for the redirect request, then the resulting t_starttransfer should
  * be 3 seconds. */
-static CURLcode test_unit1399(char *arg)
+static CURLcode test_unit1399(const char *arg)
 {
   UNITTEST_BEGIN_SIMPLE