tests: add cygwin feature

docs/tests/FILEFORMAT.md

@@ -455,6 +455,7 @@ Features testable here are:
 - `codeset-utf8`. If the running codeset is UTF-8 capable.
 - `cookies`
 - `crypto`
+- `cygwin`
 - `Debug`
 - `digest`
 - `DoH`

tests/runtests.pl

@@ -568,6 +568,9 @@ sub checksystemfeatures {
                 $pwd = sys_native_current_path();
                 $feature{"win32"} = 1;
             }
+            if($curl =~ /cygwin|msys/i) {
+                $feature{"cygwin"} = 1;
+            }
             if($libcurl =~ /\sschannel\b/i) {
                 $feature{"Schannel"} = 1;
                 $feature{"SSLpinning"} = 1;