Add and use tests_stable.txt with tests that are known to pass

@markhamstra

Disabling the following tests for now. Using a new file, tests_stable.txt, so the original tests_pass.txt is unmodified. Also the "unstable" tests are marked as "#unstable: " in tests_stable.txt.

1 Failed query: lock1.q

2 Failed query: lock2.q

3 Failed query: lock3.q

4 Failed query: lock4.q

5 Failed query: symlink_text_input_format.q

6 Failed query: udaf_ngrams.q

7 Failed query: udaf_percentile_approx.q

8 Failed query: udf_substr.q

9 Failed query: udf_union.q