tests: Run tests on Linux CI

[euanh]

The EndToEnd tests cannot currently be run:

• they deadlock when run under swift test (#143)
• the CI requires the use of an HTTP proxy to download from the Internet, but SDK generator does not support proxies (#145)

Until now the tests were not compiled on macOS, and the tests were not run on Linux, so CI runs did not deadlock. This PR marks the tests with XCTSkip to prevent them running in CI or when run locally; future PRs will loosen these restrictions.

testQueryEncoding failed on main because Foundation.URL had changed to FoundationEssentials.URL, causing cache hashes to be different. This PR ensures backward compatibility by overriding the type name used when hashing URLs to be Foundation.URL.

Fixes #45

[euanh]

@swift-ci test

[euanh]

Expect the first run to fail testQueryEncoding on main because Foundation.URL is now called FoundationEssentials.URL.

[euanh]

@swift-ci test

[euanh]

@swift-ci test

[euanh]

@swift-ci test