refactor(tvix/eval): move nix_tests to separate module r/8180

Gate this behind the nix_tests feature flag (which wasn't applied
consistently).

Also, at least right now all of these use StdIO internally, either by
creating it on their own, or through the `eval_test` helper.

Once we have some proper classification system for tests we can probably
split this up further, but for now only run them if the impure feature
is enabled.

Change-Id: I3236cf09b3391585df99073367c4e4832c5e7898
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11723
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>

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