test(download): serialize tests with proxy-sensitive URLs

[cuviper]

There was already a guard for tests that set a proxy variable in the
environment, but this is also relevant for tests that would implicitly
read those variables while downloading a local serve_file URL.

[cuviper]

Without this change, I could pretty reliably induce this race by narrowing the testsuite, so they're more likely to run in parallel.

$ cargo test --features test --lib -- download::tests::
Finished `test` profile [unoptimized + debuginfo] target(s) in 0.07s
Running unittests src/lib.rs (target/debug/deps/rustup-d6143b76edd93be0)
running 6 tests
test download::tests::curl::partially_downloaded_file_gets_resumed_from_byte_offset ... ok
test download::tests::curl::callback_gets_all_data_as_if_the_download_happened_all_at_once ... FAILED
test download::tests::reqwest::resume_partial_from_file_url ... ok
test download::tests::reqwest::callback_gets_all_data_as_if_the_download_happened_all_at_once ... FAILED
test download::tests::reqwest::socks_proxy_request ... FAILED
test download::tests::reqwest::read_basic_proxy_params ... ok
failures:
---- download::tests::curl::callback_gets_all_data_as_if_the_download_happened_all_at_once stdout ----
thread 'download::tests::curl::callback_gets_all_data_as_if_the_download_happened_all_at_once' panicked at src/download/tests.rs:85:14:
Test download failed: error during download
Caused by:
[97] proxy handshake error (connection to proxy closed)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
---- download::tests::reqwest::callback_gets_all_data_as_if_the_download_happened_all_at_once stdout ----
thread 'download::tests::reqwest::callback_gets_all_data_as_if_the_download_happened_all_at_once' panicked at src/download/tests.rs:252:14:
Test download failed: error downloading file
Caused by:
0: error sending request
1: client error (Connect)
2: socks connect error: unexpected end of file
---- download::tests::reqwest::socks_proxy_request stdout ----
thread 'download::tests::reqwest::socks_proxy_request' panicked at src/download/tests.rs:187:13:
assertion `left == right` failed
left: 3
right: 1
failures:
download::tests::curl::callback_gets_all_data_as_if_the_download_happened_all_at_once
download::tests::reqwest::callback_gets_all_data_as_if_the_download_happened_all_at_once
download::tests::reqwest::socks_proxy_request
test result: FAILED. 3 passed; 3 failed; 0 ignored; 0 measured; 114 filtered out; finished in 0.01s
error: test failed, to rerun pass `--lib`

Note that the socks_proxy_request saw 3 requests when it expected 1, because the two callback tests were also using that proxy environment variable.

[rami3l]

@cuviper Thanks a lot for your investigation! This does look like one of the reasons why Rustup's CI is not as stable as expected. I'll have a look at your patch real soon :)

[rami3l]

LGTM!

Nit: Before merging, it'd be probably better to change the commit message to "all download tests" instead of just "all tests"... I know by "proxy-sensitive URLs" it implies download tests, but it looks a bit misleading at the first glance 💦 Many thanks in advance I decided to do that myself, never mind :)

[djc]

Nice, thanks!