Testsuite fails when built without std feature.

[plugwash]

Debian's CI runs tests with "--all-targets" and with various feature combinations. We found that crossbeam-epoch's testsuite fails when the "std" feature is not enabled.

https://ci.debian.net/data/autopkgtest/testing/amd64/r/rust-crossbeam-epoch/21986900/log.gz

I ground through the issues until I got the testsuite to pass, the patch can be found at

https://salsa.debian.org/rust-team/debcargo-conf/-/blob/master/src/crossbeam-epoch/debian/patches/fix-tests-no-default-features.patch

[taiki-e]

This is semi-intentional and is fine since we don't have mutually exclusive features. See rust-lang/futures-rs#2216 for more. (Given how the cargo v1 resolver we use works, there isn't even a way to reliably test cases where specific (additive) features are disabled (dev or build dependencies may enable the features, and it will be shared with normal dependencies). Our nightly feature is not strictly additive, but our CI tests both cases where it is enabled and where it is not.)

If build errors of the test suite are a problem, I would accept PR to add cfg(feature = "std") to all tests at the module level.