proxy: Use environment-libproxy as the as-installed test for libproxy

Previously the .test file was generated with a reference to ${installed_tests_dir}/environment, which is only built (from the same source code) when the module for proxy configuration from environment variables (without using libproxy) is enabled.

The build-time test already correctly ran environment-libproxy.

Resolves: #208 (closed)
Bug-Debian: https://bugs.debian.org/1031166