Refactor regression tests of pg_rewind

All the modes of tests are refactored and combined into a single script
for clarity, script that can be invocated with a simple "make check"
command, making it transparent to user.
Mention in README how to launch regression tests
Add expected test suite output

This was forgotten by last commit because of .gitignore that forbid
its inclusion.
Prepare pg_rewind for 9.4 development
Fix GIN split record parsing for 9.4.