When implementing async CMAP, the spec tests occasionally failed on 4.2 / async-std variants. We attempted to reproduce this after adding more debugging information to the assertions, but couldn't get it to fail again. Whenever the test fails in the future, we should investigate it.

failure patches:
https://evergreen.mongodb.com/version/5e62ab25d6d80a1d9a8e995b
https://evergreen.mongodb.com/version/5e618578a4cf47624941e824
 

failure log output:

2020/03/06 16:05:05.063] ---- cmap::test::cmap_spec_tests stdout ----
 [2020/03/06 16:05:05.063] thread 'cmap::test::cmap_spec_tests' panicked at 'assertion failed: `(left == right)`
 [2020/03/06 16:05:05.063]   left: `1`,
 [2020/03/06 16:05:05.063]  right: `3`', src/cmap/test/mod.rs:134:9