Enable autotools-based testing

Automakes provides facilities to run tests and collect their
results. This can be enabled simply by adding a TESTS setting which
contains the names of the test programs to be run.

All of the tests in the "tests/" folder are now enabled except for
testgdi, which requires interaction with the user.

Commit licensed under MIT/X11.
Fix testbits

The testbits test failure indicates an issue in the definition and use
of the BitmapData structure. Document the issue and work around it in
the test, to allow the test to actually check the behaviour of the
implementation.

Also see https://bugzilla.xamarin.com/show_bug.cgi?id=20682

Commit licensed under MIT/X11.
Merge pull request #21 from ranma42/tests

Enable testing