runtests: make 'c-ares' a "feature" to depend on

... also added to the docs.

tests/FILEFORMAT.md

@@ -333,6 +333,7 @@ SKIPPED.
 Features testable here are:
 
 - `alt-svc`
+- `c-ares`
 - `cookies`
 - `crypto`
 - `debug`

tests/runtests.pl

@@ -2762,6 +2762,7 @@ sub compare {
 }
 
 sub setupfeatures {
+    $feature{"c-ares"} = $has_cares;
     $feature{"alt-svc"} = $has_altsvc;
     $feature{"HSTS"} = $has_hsts;
     $feature{"brotli"} = $has_brotli;