impl(testing_util): include algorithm header

`google/cloud/testing_util/command_line_parsing.cc` appears to use
`std::transform` without including `<algorithm>`. We encountered this
in Nixpkgs; you can see some example build logs at the bottom of
https://hydra.nixos.org/build/323365915/log. I'm not *quite* sure of the
root cause of this error appearing now (it may be related to GCC 15),
but I believe this is nonetheless a correct change.

Author: whispersofthedawn

google/cloud/testing_util/command_line_parsing.cc

@@ -16,6 +16,7 @@
 #include "google/cloud/internal/throw_delegate.h"
 #include "absl/strings/match.h"
 #include "absl/time/time.h"
+#include <algorithm>
 #include <cctype>
 #include <iomanip>
 #include <sstream>