support/testing: add test for check-package

Check the basic usage for check-package.

It can be called using either absolute path, relative path or from PATH.
Files to be checked can be passed with either absolute path or relative
path (also including files in the current directory).

Also check it ignores some special files when checking intree files,
i.e. package/pkg-generic.mk, while still generating warnings for out-of-tree
files when called with -b.
In order to allow the later, add an empty line to the Config.in in the
br2-external being tested so the script does generate a warning.

Catches bug #11271.

More tests can be added later, for example compatibility to Python 3.

Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
[Arnout: wrap at 80 columns where appropriate; merge into a single
class.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

support/testing/tests/utils/br2-external/Config.in

@@ -0,0 +1 @@
+

support/testing/tests/utils/br2-external/external.desc

@@ -0,0 +1 @@
+name: CHECK_PACKAGE