use local squid docker instance for proxy testing

use httpbingo for HTPP tests

This service is based on httpbin-go which seems to be better maintained
and should be easy to set up for ourselves if needed.

This also fixes a warning when the client

use httpbingo for HTPP tests

This service is based on httpbin-go which seems to be better maintained
and should be easy to set up for ourselves if needed.

This also fixes a warning when the client is reused with the
max_bodysize mechanism

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add needed type hints for phpunit8

This will break a lot of plugin tests, but can't be avoided

run HTTP tests against EU server

this hopefully helps with recent problems of spradically failing tests.

fixed HTTPS proxy tests, our tests now run on httpbin.org

This also reverses the order of crypto protocols tried again. Using TLS
first again. related to #915

use correct host in proxy tests

HTTPClient: correctly abort a proxy connection

if a needed CONNECT tunnel fails

support CONNECT for tunneling SSL via HTTP proxies FS#2431

The included test cases currently expect a squid at localhost:3128