curl: --test-duphandle in debug builds runs "duphandled"

Using this option (only available in debug builds) makes curl always
call curl_easy_duphandle() on the handle before using it.

To help us catch curl_easy_duphandle() mistakes better.

Add a CI job using this.

Bonus: the previous runtests option -e is now also supported as
--test-event

Closes #15504

src/tool_getparam.h

@@ -267,6 +267,7 @@ typedef enum {
   C_TCP_FASTOPEN,
   C_TCP_NODELAY,
   C_TELNET_OPTION,
+  C_TEST_DUPHANDLE,
   C_TEST_EVENT,
   C_TFTP_BLKSIZE,
   C_TFTP_NO_OPTIONS,