tests/test-absorb-strip.t

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.

Do not strip innocent children. See https://bitbucket.org/facebook/hg-experimental/issues/6/hg-absorb-merges-diverged-commits

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > absorb=
  > drawdag=$TESTDIR/drawdag.py
  > EOF

  $ hg init
  $ hg debugdrawdag << EOF
  > E
  > |
  > D F
  > |/
  > C
  > |
  > B
  > |
  > A
  > EOF

  $ hg up E -q
  $ echo 1 >> B
  $ echo 2 >> D
  $ hg absorb -a
  warning: orphaned descendants detected, not stripping 112478962961, 26805aba1e60
  saved backup bundle to * (glob)
  2 of 2 chunk(s) applied

  $ hg log -G -T '{desc}'
  @  E
  |
  o  D
  |
  o  C
  |
  o  B
  |
  | o  F
  | |
  | o  C
  | |
  | o  B
  |/
  o  A