tests/test-empty-dir.t

run-tests: create classes for representing tests

Currently, the state for an individual test is scattered across a number
of functions and variables. This patch begins a process of isolating a
single test's state into instances of a class. It does this by
establishing a new Test base class and child classes for Python tests
and T tests. The class currently has a run() API that proxies into the
existing "runner" functions. Upcoming patches will move the logic for
each test type into the class.

  $ hg init

  $ echo 123 > a
  $ hg add a
  $ hg commit -m "first" a

  $ mkdir sub
  $ echo 321 > sub/b
  $ hg add sub/b
  $ hg commit -m "second" sub/b

  $ cat sub/b
  321

  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ cat sub/b 2>/dev/null || echo "sub/b not present"
  sub/b not present

  $ test -d sub || echo "sub not present"
  sub not present