clang-tidy: fix issues found with build-fuzzing

- curl_sha512_256: add missing, drop redundant, parentheses.
- doh: drop redundant returns.
- url: add missing parentheses.
- vtls: fix unused const variables.
- tests/unit: fix missing header with clang-tidy and !threaded-resolver.
  Follow-up to 57ff2d6c91 #20106

Closes #20774

tests/unit/unit1607.c

@@ -25,6 +25,7 @@
 
 #include "urldata.h"
 #include "connect.h"
+#include "curl_addrinfo.h"
 
 static CURLcode t1607_setup(void)
 {

tests/unit/unit1609.c

@@ -25,6 +25,7 @@
 
 #include "urldata.h"
 #include "connect.h"
+#include "curl_addrinfo.h"
 
 static CURLcode t1609_setup(void)
 {

tests/unit/unit2600.c

@@ -46,6 +46,7 @@
 #include "cf-ip-happy.h"
 #include "multiif.h"
 #include "select.h"
+#include "curl_addrinfo.h"
 #include "curl_trc.h"
 
 static CURLcode t2600_setup(CURL **easy)