Use dup2() to redirect stderr in the common/error unit test.

The assignment was obviously not kosher but seemed fine for a test in short-lived fork. However, musl libc defines stderr as read-only so this trick won't work.

Instead use dup2(), which is pretty clearly the correct way to do this.

test/src/module/common/errorTest.c

@@ -416,7 +416,7 @@ testRun(void)
             HRN_FORK_CHILD_BEGIN(.expectedExitStatus = UnhandledError.code)
             {
                 // Redirect stderr to stdout (we do not care about the output here since coverage will tell us we hit the code)
-                stderr = stdout;
+                dup2(fileno(stdout), fileno(stderr));
 
                 THROW(TestChildError, "does not get caught!");
             }