CodingStyle: verify that business template re-exporting is stable

re-exporting a business template that was just installed should not be
different.

By having a test reporting diffs in export, we can detect changes in ERP5 or
dependencies which would cause the business template format to become different.
The primary reason for doing this was to make sure that on the way to Zope4 we
don't have changes in exported data, especially lossly changes that would not
be detected in test suite, but this is also good because it helps detecting
changes that cause a difference in output - then if we introduce a change that
cause different business template export, we also have to make a change to
re-export everything to new format, which helps keeping the git history cleaner
because there's one big commit with all the changes instead of having many small
commits. This is also good because it detects mistakes when modifying business
templates files directly with a text editor.

The test can be configured to ignore some paths, using fnmatch pattern (which
unlike business patterns do not support ** for recursive match). This is also
helpful when using this test case to check customer configuration business
templates, where we may want to ignore some PathTemplateItem for convenience.