tests: bundle http clients, de-dupe, enable for MSVC

To make building the http client tests faster, with no duplication, by
using the build method that other test binaries already use.

The difference compared to other tests is that these don't use internal
libcurl headers or code. With the exception of `curl_config.h`, for
a feature macro.

Before this patch, these tests were built like examples.

Also:
- de-duplicate code and give unique names to colliding symbols.
- add local getopt implementation and enable all code for MSVC.
  Adapted for curl via Public Domain source:
  4e618ef782/getopt.h
  Credits-to: Christopher Wellons
  Thanks!

Closes #17627

scripts/mk-unity.pl

@@ -76,7 +76,7 @@ if($any_test) {
 my $tlist = "";
 
 foreach my $src (@src) {
-    if($src =~ /([a-z0-9]+)\.c$/) {
+    if($src =~ /([a-z0-9_]+)\.c$/) {
         my $name = $1;
         if($embed) {
             my $fn = $src;