libphobos: Add script for extracting unittests from phobos

This script parses all unittests annotated with three slashes (`///')
and extracts them into a standalone test case.  The intended use is for
generating inexpensive tests to be ran for the phobos testsuite.

libphobos/ChangeLog:

	* scripts/.gitignore: Add tests_extractor.
	* scripts/README: Document tests_extractor.d.
	* scripts/tests_extractor.d: New file.