tests/test-xdg.t

rhg: use a release-mode executable in tests

This allows the rhg build for test-rhg.t to share compiled dependencies
such as hg-core with the hg-cpython build for other tests.

For context, my wrapper script for the typical edit-compile-test
cycle now looks like this:

    (cd rust && cargo +nightly-2020-10-04 fmt)
    (cd rust && cargo build --release -p rhg)
    make --silent local PURE=--rust
    python test/run-tests.py --local "$@"

Differential Revision: https://phab.mercurial-scm.org/D9728

#if no-windows no-osx

  $ mkdir -p xdgconf/hg
  $ echo '[ui]' > xdgconf/hg/hgrc
  $ echo 'username = foobar' >> xdgconf/hg/hgrc
  $ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
  $ unset HGRCPATH
  $ hg config ui.username 2>/dev/null
  foobar

#endif