Due to the way cargo works .veil.toml can't impact other crates
consistently.

This reverts commit 69dd762.

This is a simpler replacement for the environmental awareness feature.

This is neccessary because rust runs all tests in the same process,
leading to tests failing randomly, because of other tests modifying the
global state. There is some proposal on the rust compiler-team github
repo but until that gets stabilized we have to use this hack instead
rust-lang/compiler-team#508

Co-authored-by: William <[email protected]>

Ensure that the use of this feature requires a very explicit opt-in from
the user.

* Rename it to veil-tests-disable-redaction
* Set publish = false
* Change the outer line doc comment describing the test into a inner
  line doc comment

This avoids repeating that the disable function needs the toggle feature
flag which makes it clearer that the VEIL_DISABLE_REDACTION envar also
needs the feature flag

Co-authored-by: William <[email protected]>

Make it clear that to skip redacting data the user needs to either call
the disable function *or* set the VEIL_DISABLE_REDACTION variable, not
both.