Add flag to build package passthru.tests

[B4dM4n]

This PR adds a --tests flag to build passthru.tests for all packages, as discussed in #77.

The tests to be build can be filtered with -p, -P and similar, like all other packages.

$ nixpkgs-review pr --tests 303033
...
Link to currently reviewing PR:
https://github.com/NixOS/nixpkgs/pull/303033

1 tests built:
netbird.passthru.tests.netbird

2 packages built:
netbird netbird-ui
...

$ nixpkgs-review pr --tests --package-regex 'netbird.+' 303033
...
1 tests built:
netbird.passthru.tests.netbird

1 package built:
netbird-ui
...

Since they are classified as tests, they can also be enabled individually with -p, without specifying --tests:

$ nixpkgs-review pr -p netbird.passthru.tests.netbird 303033
...
1 tests built:
netbird.passthru.tests.netbird
...

Fixes: #77

[Mic92]

Sorry for the late reply. I think because of the potential large memory consumption of nixos tests, we need to evaluate them isolation as they will otherwise make the build potentially go out of memory, i.e. if you try to have a large package set rebuild.
Therefore I would suggest to evaluate those packages ahead of time i.e. using nix-instantiate and add them to the final nix build. Of course a better design would be some sort of pipe-lining where already evaluated packages gets build as the rest of the evaluation is still in progress. But this probably requires bigger refactorings and could implemented at a later point similar to nix-fast-build. When using nix-instantiate also add a gcroot with --add-root to make sure that derivations are not garbage collected before hand.