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.

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

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tests: use %LOGDIR to refer to the log directory

This will allow it be set dynamically.

Ref: #10818

test{440,441,493,977}: add "HTTP proxy" keywords

... so the tests can be automatically skipped when
using an external proxy like Privoxy.

Closes #8959

test440/441: verify HSTS with trailing dots