units: tidy up begin/end blocks

- use `UNITTEST_BEGIN_SIMPLE`/`UNITTEST_END_SIMPLE` where missing.
- drop redundant `(void)arg;` where using `UNITTEST_BEGIN*`.
- unit1636: drop redundant block after `UNITTEST_BEGIN*`.
- unit1609: fix typo in comment.
- unit1627: merge to `if`s.

Closes #21715

tests/unit/unit3301.c

@@ -134,11 +134,9 @@ static CURLcode test_unit3301(const char *arg)
 }
 
 #else
-
 static CURLcode test_unit3301(const char *arg)
 {
   UNITTEST_BEGIN_SIMPLE
-  (void)arg;
   UNITTEST_END_SIMPLE
 }
 #endif /* USE_THREADS */