Add more spellcheck selftests

Add more spellcheck selftests

gcc/ChangeLog:
	* selftest-run-tests.c (selftest::run_tests): Call
	selftest::spellcheck_tree_c_tests.
	* selftest.h (selftest::spellcheck_tree_c_tests): New decl.
	* spellcheck-tree.c: Include selftest.h and stringpool.h.
	(selftest::test_find_closest_identifier): New function.
	(selftest::spellcheck_tree_c_tests): New function.
	* spellcheck.c (selftest::test_find_closest_string): Verify that
	the order of the vec does not affect the results for this case.
	(selftest::test_data): New array.
	(selftest::test_metric_conditions): New function.
	(selftest::spellcheck_c_tests): Add a test of case-comparison.
	Call selftest::test_metric_conditions.

From-SVN: r237466