Various unit tests fail, possibly due to previous test not resetting prefix

When running all unit tests, various ones fail with a message like the following (this is the first):

Skip: lib/simpletest/testblocklib_blockmanager.php / ?
Unit test "moodle_block_manager_test_saving_loading" of type UnitTestCaseUsingDatabase skipped. Must define different, non-conflicting $CFG->unittestprefix to be runnable.

This may indicate that a previous test did not correctly reset the database prefix.