tests: build non-debug unit tests with autotools, run them

Before this patch, autotools disabled building unit tests for
non-debug-enabled (`DEBUGBUILD`) builds. runtests skipped running this
combination, though they were built in cmake builds. There seems to be
no technical reason anymore for these restrictions. This patch removes
them, allowing to build and run unit tests for non-debug-enabled builds.

To improve unit test build and run coverage.

- autotools: do not disable building unit tests for non-debug-enabled
  build. Bringing behavior closer to cmake builds. (There are still
  exceptions in autotools, something for another PR)

- runtests: run unit tests for non-debug-enabled builds.
  It extends coverage by 50 tests.

- `lib/altsvc.c`: fix to include `CURL_TIME` support in libcurlu, for
  unit tests. It fixes test 1654, and syncs `CURL_TIME` behavior with
  test 1660 and `lib/hsts.c`.

Ref: 10a7d05be3
Ref: fc8e0dee30 #13694
Ref: 99f78cbf6e #16770

Closes #16771

lib/altsvc.c

@@ -438,7 +438,7 @@ static void altsvc_flush(struct altsvcinfo *asi, enum alpnid srcalpnid,
   }
 }
 
-#ifdef DEBUGBUILD
+#if defined(DEBUGBUILD) || defined(UNITTESTS)
 /* to play well with debug builds, we can *set* a fixed time this will
    return */
 static time_t altsvc_debugtime(void *unused)