Auto merge of #8168 - ecstatic-morse:future-proof-test, r=alexcrichton
Auto merge of #8168 - ecstatic-morse:future-proof-test, r=alexcrichton

Clear `RUSTDOCFLAGS` before running tests

It turns out that this test will fail if `RUSTDOCFLAGS` is set in the test runner. This was found in
https://github.com/rust-lang/rust/pull/71458#issuecomment-619101858 when that PR attempted to set the proper `RUSTDOCFLAGS` to enable `./x.py doc --stage 0 src/libstd`. Run the test with no `RUSTDOCFLAGS` so that the config file always gets used.

rust-lang/cargo,rustyhorde/cargo,eminence/cargo,tsurai/cargo,tsurai/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,tshepang/cargo,tshepang/cargo,rust-lang/cargo,tshepang/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,rustyhorde/cargo,tsurai/cargo,tsurai/cargo,tsurai/cargo,eminence/cargo,nipunn1313/cargo,tshepang/cargo