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

.github/workflows/linux.yml

@@ -248,7 +248,12 @@ jobs:
           - name: event-based
             install_packages: libssh-dev
             configure: --enable-debug --disable-shared --disable-threaded-resolver --with-libssh --with-openssl
-            tflags: -n -e '!TLS-SRP'
+            tflags: -n --test-event '!TLS-SRP'
+
+          - name: duphandle
+            install_packages: libssh-dev
+            configure: --enable-debug --disable-shared --disable-threaded-resolver --with-libssh --with-openssl
+            tflags: -n --test-duphandle '!TLS-SRP'
 
           - name: rustls valgrind
             install_packages: valgrind