Some tests in read_repair_test are flakey

Since being refactored for CASSANDRA-14134, test_alter_rf_and_run_read_repair and test_read_repair_chance in read_repair_test.TestReadRepair are flakey and regularly fail on all branches. The problem is that the inital setup of these two tests doesn't explicitly set the read_repair_chance or dclocal_read_repair_chance properties on the test table. As a consequence, read repairs are sometimes probabilistically triggered and query results don't match the expectations.