testing: update description of Setenv
testing: update description of Setenv

Add the description of Setenv that it cannot use if the test have
parallel ancestors.

Fixes #55128

Change-Id: Ia5a1deaa1a3116d1ebb439600a7d316c7d155412
Reviewed-on: https://go-review.googlesource.com/c/go/+/434115
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go