rust-cpython: prepare for writing tests that require libpython
What I wanted is to disable the "cpython/extension-module<ver>" feature
while building tests executable, but that seems not doable. Instead,
this patch adds new features dedicated for tests.
The make rule is extracted so that we can easily run cargo tests.
Added a minimal test of ref-sharing as an example. More tests will follow.
#require vcr
$ cat >> $HGRCPATH <<EOF
> [extensions]
> phabricator =
> EOF
$ hg init repo
$ cd repo
$ cat >> .hg/hgrc <<EOF
> [phabricator]
> url = https://phab.mercurial-scm.org/
> callsign = HG
>
> [auth]
> hgphab.schemes = https
> hgphab.prefix = phab.mercurial-scm.org
> # When working on the extension and making phabricator interaction
> # changes, edit this to be a real phabricator token. When done, edit
> # it back. The VCR transcripts will be auto-sanitised to replace your real
> # token with this value.
> hgphab.phabtoken = cli-hahayouwish
> EOF
$ VCR="$TESTDIR/phabricator"