bug#46650: Extending package transformation options

[Carl Dong]

Hi Guix!

TL;DR Shall we add a `--without-tests` option (or something similar) for `guix 
time-machine`?

Our release workflow in Bitcoin is planned to shift over to Guix for the next 
release, and as such we have a lot more testers who are running Guix as we 
speak on their machines! Exciting!

Our build workflow basically boils down to something like:

--8<---------------cut here---------------start------------->8---
$ guix time-machine --commit=<pinned-guix> … \
       -- environment --manifest=manifest.scm … \
          -- <build-command>
--8<---------------cut here---------------end--------------->8---

The fact that this is even possible (and allows reproducibility across time) 
speaks to the awesomeness of Guix! :-)

One of the main reasons for switching over to Guix is for its bootstrappbility 
and the ability to perform `--bootstrap --no-substitutes` builds. As mentioned 
in the gnutls ticket, performing these source-based builds can sometimes reveal 
problems with tests suites:
1. Spurious tests
2. “Expiring” tests (like the gnutls one)
3. Foreign distro problems (0zmcam9sdiag9s1qx3xvlf8a3jnm53x2-sed-4.8.drv fails 
to build on SELinux-based distros because of a test failure in 
testsuite/inplace-selinux)

I think that the package transformation options are the perfect “hack-around” 
solution for this, namely `--without-tests`. In fact, since those who 
experience these problems are building from source, it makes no difference that 
the store item would be change: they would need to build everything anyway!

However, in the case of our workflow above, there is no way to specify 
`--without-tests=` to `guix time-machine`, which insists on building the broken 
`gnutls` package and running its tests. I’ve tried isolating the command to 
just:

--8<---------------cut here---------------start------------->8---
$ guix time-machine --commit=<pinned-guix> … \
       -- --help
--8<---------------cut here---------------end--------------->8—

To make sure that it’s `guix time-machine` which is trying to build gnutls and 
not the time-machine’d `guix environment`.

I’m not too familiar with how package transformation options work internally, 
but I’m guessing all we need here is a `--without-tests` option (or something 
similar) for `guix time-machine`? Is there anything else that’s necessary to 
successfully skip tests for my workflow?

Cheers,
Carl Dong
contact@carldong.me
"I fight for the users"