Fix race in test_inherit_env

[stepancheg]

env tests set (and unset) randomly generated variables with names
starting with TEST. So in test_inherit_env between variable
capture and process spawn TEST* variables can be unset if env
tests executed concurrently. Thus TEST* variables must not be
checked in that test.

[rust-highfive]

r? @aturon

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[alexcrichton]

Could this test actually be moved to a run-pass test? Most of the env tests probably should be in the long run because they're modifying global state, but it's fine to move just this one for now.

[stepancheg]

Shouldn't tests which modify global state be moved to run-pass, and this test (which doesn't modify global state) kept where it is instead?

[alexcrichton]

Yes, depends on how much work you're willing to do. One's much easier than the other most likely

[orivej]

Could it be that test_inherit_env is still failing because it was not moved into run-pass? (https://github.com/rust-lang/rust/blob/1.26.2/src/libstd/process.rs#L1748) Seen with Rust 1.26.2 here: https://hydra.nixos.org/build/75678845/nixlog/1, and with Rust 1.25.0 here: https://hydra.nixos.org/build/74824788/nixlog/1